WebfilesValidator: whitelist mailto: URLs

Summary:
This patch teaches `WebfilesValidator` that `mailto:` URLs don’t express
webfiles dependency relationships, and so do not need to be accompanied
by a `web_library` provider. See:
<tensorflow/tensorboard#2297>

Test Plan:
Unit test added; it fails before this change and passes after it.

wchargin-branch: whitelist-mailto

java/io/bazel/rules/closure/webfiles/WebfilesValidator.java

@@ -216,6 +216,7 @@ private static boolean shouldIgnoreUri(String uri) {
         || uri.contains("//")
         || uri.startsWith("data:")
         || uri.startsWith("javascript:")
+        || uri.startsWith("mailto:")
         || uri.equals("about:blank")
         // The following are intended to filter out URLs with Polymer variables.
         || (uri.contains("[[") && uri.contains("]]"))

javatests/io/bazel/rules/closure/webfiles/WebfilesValidatorTest.java

@@ -205,6 +205,22 @@ public void dataUris_getIgnored() throws Exception {
         .isEmpty();
   }
 
+  @Test
+  public void mailtoUris_getIgnored() throws Exception {
+    save(fs.getPath("/fs/path/index.html"), "<a href=\"mailto:[email protected]\">email us</a>\n");
+    assertThat(
+            validator.validate(
+                Webfiles.newBuilder()
+                    .addSrc(WebfilesSource.newBuilder()
+                        .setPath("/fs/path/index.html")
+                        .setWebpath("/web/path/index.html")
+                        .build())
+                    .build(),
+                ImmutableList.<Webfiles>of(),
+                Suppliers.ofInstance(ImmutableList.<Webfiles>of())))
+        .isEmpty();
+  }
+
   @Test
   public void cssUrls_areRecognized() throws Exception {
     save(fs.getPath("/fs/path/index.html"), "<link rel=\"stylesheet\" href=\"index.css\">");