fix($compile): secure link[href] as a RESOURCE_URLs in $sce.

User-controlled imports or stylesheets can run script in your origin,
which warrants that we require that they are safe `RESOURCE_URL`s.

Closes #14687

BREAKING CHANGE

`link[href]` attributes are now protected via `$sce`, which prevents interpolated
values that fail the `RESOURCE_URL` context tests from being used in interpolation.

For example if the application is running at `https://docs.angularjs.org` then the
following will fail:

```
<link href="{{ 'http://mydomain.org/unsafe.css' }}" rel="stylesheet">
```

By default, `RESOURCE_URL` safe URLs are only allowed from the same domain and protocol
as the application document.

To use URLs from other domains and/or protocols, you may either whitelist them or
wrap it into a trusted value by calling `$sce.trustAsResourceUrl(url)`.

src/ng/compile.js

@@ -2971,6 +2971,8 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
       // maction[xlink:href] can source SVG.  It's not limited to <maction>.
       if (attrNormalizedName === "xlinkHref" ||
           (tag === "form" && attrNormalizedName === "action") ||
+          // links can be stylesheets or imports, which can run script in the current origin
+          (tag === "link" && attrNormalizedName === "href") ||
           (tag !== "img" && (attrNormalizedName === "src" ||
                             attrNormalizedName === "ngSrc"))) {
         return $sce.RESOURCE_URL;

test/ng/compileSpec.js

@@ -10171,6 +10171,7 @@ describe('$compile', function() {
           "loading resource from url not allowed by $sceDelegate policy.  URL: javascript:doTrustedStuff()");
     }));
 
+
     it('should pass through $sce.trustAs() values in action attribute', inject(function($compile, $rootScope, $sce) {
       /* jshint scripturl:true */
       element = $compile('<form action="{{testUrl}}"></form>')($rootScope);
@@ -10181,6 +10182,36 @@ describe('$compile', function() {
     }));
   });
 
+  describe('link[href]', function() {
+    it('should reject invalid RESOURCE_URLs', inject(function($compile, $rootScope) {
+      element = $compile('<link href="{{testUrl}}" rel="stylesheet" />')($rootScope);
+      $rootScope.testUrl = "https://evil.example.org/css.css";
+      expect(function() { $rootScope.$apply(); }).toThrowMinErr(
+          "$interpolate", "interr", "Can't interpolate: {{testUrl}}\nError: [$sce:insecurl] Blocked " +
+          "loading resource from url not allowed by $sceDelegate policy.  URL: " +
+          "https://evil.example.org/css.css");
+    }));
+
+    it('should accept valid RESOURCE_URLs', inject(function($compile, $rootScope, $sce) {
+      element = $compile('<link href="{{testUrl}}" rel="stylesheet" />')($rootScope);
+
+      $rootScope.testUrl = "./css1.css";
+      $rootScope.$apply();
+      expect(element.attr('href')).toContain('css1.css');
+
+      $rootScope.testUrl = $sce.trustAsResourceUrl('https://elsewhere.example.org/css2.css');
+      $rootScope.$apply();
+      expect(element.attr('href')).toContain('https://elsewhere.example.org/css2.css');
+    }));
+
+    it('should accept valid constants', inject(function($compile, $rootScope) {
+      element = $compile('<link href="https://elsewhere.example.org/css2.css" rel="stylesheet" />')($rootScope);
+
+      $rootScope.$apply();
+      expect(element.attr('href')).toContain('https://elsewhere.example.org/css2.css');
+    }));
+  });
+
   if (!msie || msie >= 11) {
     describe('iframe[srcdoc]', function() {
       it('should NOT set iframe contents for untrusted values', inject(function($compile, $rootScope, $sce) {