Replace compile time guard with runtime flag in blink and remove...

Replace compile time guard with runtime flag in blink and remove ENABLE_FAST_MOBILE_SCROLLING in gyp file.

The effort needs to be done in 3 steps:
1. Create the runtime flag interface in blink;
2. Enable the flag for Android from chromium even it does not do anything in blink;
3. Replace compile time guard with runtime flag in blink and remove ENABLE_FAST_MOBILE_SCROLLING in gyp file.

This is the 3rd set of changes.

BUG=381391

Review URL: https://codereview.chromium.org/323123004

git-svn-id: svn://svn.chromium.org/blink/trunk@175967 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/build/features.gypi

@@ -52,7 +52,6 @@
       }],
       ['OS=="android"', {
         'feature_defines': [
-          'ENABLE_FAST_MOBILE_SCROLLING=1',
           'ENABLE_MEDIA_CAPTURE=1'
         ],
       }, { # OS!="android"

third_party/WebKit/Source/config.gni

@@ -65,7 +65,6 @@ if (use_concatenated_impulse_responses) {
 
 if (is_android) {
   feature_defines_list += [
-    "ENABLE_FAST_MOBILE_SCROLLING=1",
     "ENABLE_MEDIA_CAPTURE=1",
   ]
 } else {

third_party/WebKit/Source/core/rendering/RenderBoxModelObject.cpp

@@ -1005,15 +1005,15 @@ void RenderBoxModelObject::calculateBackgroundImageGeometry(const RenderLayerMod
     // FIXME: transforms spec says that fixed backgrounds behave like scroll inside transforms.
     bool fixedAttachment = fillLayer->attachment() == FixedBackgroundAttachment;
 
-#if ENABLE(FAST_MOBILE_SCROLLING)
-    if (view()->frameView() && view()->frameView()->shouldAttemptToScrollUsingFastPath()) {
+    if (RuntimeEnabledFeatures::fastMobileScrollingEnabled()
+        && view()->frameView()
+        && view()->frameView()->shouldAttemptToScrollUsingFastPath()) {
         // As a side effect of an optimization to blit on scroll, we do not honor the CSS
         // property "background-attachment: fixed" because it may result in rendering
         // artifacts. Note, these artifacts only appear if we are blitting on scroll of
         // a page that has fixed background images.
         fixedAttachment = false;
     }
-#endif
 
     if (!fixedAttachment) {
         geometry.setDestRect(snappedPaintRect);

third_party/WebKit/Source/core/rendering/RenderObject.cpp

@@ -2235,14 +2235,13 @@ void RenderObject::styleWillChange(StyleDifference diff, const RenderStyle& newS
 
     if (view()->frameView()) {
         bool shouldBlitOnFixedBackgroundImage = false;
-#if ENABLE(FAST_MOBILE_SCROLLING)
-        // On low-powered/mobile devices, preventing blitting on a scroll can cause noticeable delays
-        // when scrolling a page with a fixed background image. As an optimization, assuming there are
-        // no fixed positoned elements on the page, we can acclerate scrolling (via blitting) if we
-        // ignore the CSS property "background-attachment: fixed".
-        shouldBlitOnFixedBackgroundImage = true;
-#endif
-
+        if (RuntimeEnabledFeatures::fastMobileScrollingEnabled()) {
+            // On low-powered/mobile devices, preventing blitting on a scroll can cause noticeable delays
+            // when scrolling a page with a fixed background image. As an optimization, assuming there are
+            // no fixed positoned elements on the page, we can acclerate scrolling (via blitting) if we
+            // ignore the CSS property "background-attachment: fixed".
+            shouldBlitOnFixedBackgroundImage = true;
+        }
         bool newStyleSlowScroll = !shouldBlitOnFixedBackgroundImage && newStyle.hasFixedBackgroundImage();
         bool oldStyleSlowScroll = m_style && !shouldBlitOnFixedBackgroundImage && m_style->hasFixedBackgroundImage();