Linux/CrOS smooth scrolling support - behind a flag.

build/features_override.gypi

Index: build/features_override.gypi
diff --git a/build/features_override.gypi b/build/features_override.gypi
index 0fdf439f476530392f48626df25d6f7abd5eda54..c968afcb62c93574bcd546f9357857afdc6f84b5 100644
--- a/build/features_override.gypi
+++ b/build/features_override.gypi
@@ -83,12 +83,14 @@
     'variables': {
       'use_accelerated_compositing%': 1,
       'use_threaded_compositing%': 0,
+      'enable_smooth_scrolling%': 0,
       'enable_svg%': 1,
       'enable_touch_events%': 1,
       'use_skia_gpu%': 0,
     },
     'use_accelerated_compositing%': '<(use_accelerated_compositing)',
     'use_threaded_compositing%': '<(use_threaded_compositing)',
+    'enable_smooth_scrolling%': '<(enable_smooth_scrolling)',
     'enable_svg%': '<(enable_svg)',
     'enable_touch_events%': '<(enable_touch_events)',
     'conditions': [
@@ -125,6 +127,11 @@
           'ENABLE_REGISTER_PROTOCOL_HANDLER=1',
         ],
       }],
+      ['enable_smooth_scrolling==1', {
+        'feature_defines': [
+          'ENABLE_SMOOTH_SCROLLING=1',
+        ],
+      }],
     ],
     # TODO: If the need arises, create a mechanism that will intelligently
     # merge the lists rather than replace one with the other. This may