Add missing flags to the iframe sandbox supported list.

Add missing flags to the iframe sandbox supported list, and add tests.
Also remove the runtime config for 'allow-top-navigation-by-user-activation' since the feature has shipped a long time ago.

Bug: 662506
Change-Id: I458210f6133ddd5e30b507214b003ff0fb596761
Reviewed-on: https://chromium-review.googlesource.com/965843
Commit-Queue: Bin Lu <binlu@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543733}

third_party/WebKit/LayoutTests/fast/dom/HTMLIFrameElement/sandbox-feature-detection.html

@@ -24,5 +24,6 @@
         assert_true(iframe.sandbox.supports("allow-same-origin"));
         assert_true(iframe.sandbox.supports("allow-scripts"));
         assert_true(iframe.sandbox.supports("allow-top-navigation"));
+        assert_true(iframe.sandbox.supports("allow-top-navigation-by-user-activation"));
     }, "Make sure that sandbox based feature detection is working");
 </script>

third_party/WebKit/Source/core/html/HTMLIFrameElementSandbox.cpp

@@ -11,9 +11,11 @@ namespace blink {
 namespace {
 
 const char* const kSupportedSandboxTokens[] = {
-    "allow-downloads",    "allow-forms",   "allow-modals",
-    "allow-pointer-lock", "allow-popups",  "allow-popups-to-escape-sandbox",
-    "allow-same-origin",  "allow-scripts", "allow-top-navigation"};
+    "allow-downloads",      "allow-forms",
+    "allow-modals",         "allow-pointer-lock",
+    "allow-popups",         "allow-popups-to-escape-sandbox",
+    "allow-same-origin",    "allow-scripts",
+    "allow-top-navigation", "allow-top-navigation-by-user-activation"};
 
 bool IsTokenSupported(const AtomicString& token) {
   for (const char* supported_token : kSupportedSandboxTokens) {