Navigation transitions: Expose navigation transitions feature to WebRuntimeFeatures.

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

Will be followed by: https://codereview.chromium.org/389583004/

Design doc: https://docs.google.com/a/chromium.org/document/d/17jg1RRL3RI969cLwbKBIcoGDsPwqaEdBxafGNYGwiY4/edit# 
Implementation details: https://docs.google.com/a/chromium.org/document/d/1kREPtFJaeLoDKwrfmrYTD7DHCdxX1RzFBga2gNY8lyE/edit#heading=h.bng2kpmyvxq5
BUG=370696

Review URL: https://codereview.chromium.org/386943009

git-svn-id: svn://svn.chromium.org/blink/trunk@178575 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/web/WebRuntimeFeatures.cpp

@@ -160,6 +160,11 @@ void WebRuntimeFeatures::enableNavigatorContentUtils(bool enable)
     RuntimeEnabledFeatures::setNavigatorContentUtilsEnabled(enable);
 }
 
+void WebRuntimeFeatures::enableNavigationTransitions(bool enable)
+{
+    RuntimeEnabledFeatures::setNavigationTransitionsEnabled(enable);
+}
+
 void WebRuntimeFeatures::enableOrientationEvent(bool enable)
 {
     RuntimeEnabledFeatures::setOrientationEventEnabled(enable);

third_party/WebKit/public/web/WebRuntimeFeatures.h

@@ -80,6 +80,8 @@ public:
 
     BLINK_EXPORT static void enableNavigatorContentUtils(bool);
 
+    BLINK_EXPORT static void enableNavigationTransitions(bool);
+
     BLINK_EXPORT static void enableOrientationEvent(bool);
 
     BLINK_EXPORT static void enablePagePopup(bool);