Make blob scheme donwloadable through context menu

Currently filesystem and data URIs are downloadable from context menu
Blob urls should also be supported.
Context menu downloads are handled by Chrome's network stack.
So blob urls will be handled appropriately.

BUG=481497

Review URL: https://codereview.chromium.org/1126233011

Cr-Commit-Position: refs/heads/master@{#330777}

chrome/android/java/src/org/chromium/chrome/browser/UrlUtilities.java

@@ -35,7 +35,7 @@ public class UrlUtilities {
      * URI schemes that Chrome can download.
      */
     private static final HashSet<String> DOWNLOADABLE_SCHEMES = CollectionUtil.newHashSet(
-            "data", "filesystem", "http", "https");
+            "data", "filesystem", "http", "https", "blob");
 
     /**
      * URI schemes that are internal to Chrome.

chrome/android/javatests_shell/src/org/chromium/chrome/browser/UrlUtilitiesTest.java

@@ -43,6 +43,7 @@ public class UrlUtilitiesTest extends InstrumentationTestCase {
                 "https://user:pass@:awesome.com:9000/bad-scheme:#fake:"));
         assertTrue(UrlUtilities.isDownloadableScheme("http://awesome.example.com/"));
         assertTrue(UrlUtilities.isDownloadableScheme("filesystem://awesome.example.com/"));
+        assertTrue(UrlUtilities.isDownloadableScheme("blob:https%3A//awesome.example.com/"));
 
         assertFalse(UrlUtilities.isDownloadableScheme("inline:skates.co.uk"));
         assertFalse(UrlUtilities.isDownloadableScheme("javascript:alert(1)"));