Navigation transitions: Add finch trial for navigation transitions.

Thought it would be nice to keep this feature disabled by default behind the experimental flag, but be able to potentially turn it on via a finch trial from chrome.

Needs to land first: https://codereview.chromium.org/386943009/

BUG=370696

Review URL: https://codereview.chromium.org/389583004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284762 0039d316-1c4b-4281-b951-d872f2087c98

content/child/runtime_features.cc

@@ -13,6 +13,7 @@
 #if defined(OS_ANDROID)
 #include <cpu-features.h>
 #include "base/android/build_info.h"
+#include "base/metrics/field_trial.h"
 #include "media/base/android/media_codec_bridge.h"
 #endif
 
@@ -53,6 +54,11 @@ static void SetRuntimeFeatureDefaultsForPlatform() {
   WebRuntimeFeatures::enableOrientationEvent(true);
   WebRuntimeFeatures::enableFastMobileScrolling(true);
   WebRuntimeFeatures::enableMediaCapture(true);
+  // If navigation transitions gets activated via field trial, enable it in
+  // blink. We don't set this to false in case the user has manually enabled
+  // the feature via experimental web platform features.
+  if (base::FieldTrialList::FindFullName("NavigationTransitions") == "Enabled")
+    WebRuntimeFeatures::enableNavigationTransitions(true);
 #else
   WebRuntimeFeatures::enableNavigatorContentUtils(true);
 #endif  // defined(OS_ANDROID)