Replace compile time guard with runtime flag in blink and remove ENABLE_FAST_MOBILE_SCROLLING in gy…

Source/config.gni

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/ui.gni")
 
 if (is_android) {
   import("//build/config/android/config.gni")
 } else {
   # TODO(brettw) remove this once && early-out is checked in.
@@ skipping 47 matching lines @@
   # but we don't have that flag in the GN build yet.
   feature_defines_list += [ "WTF_USE_DYNAMIC_ANNOTATIONS=1" ]
 }
 
 if (use_concatenated_impulse_responses) {
   feature_defines_list += [ "WTF_USE_CONCATENATED_IMPULSE_RESPONSES=1" ]
 }
 
 if (is_android) {
   feature_defines_list += [
-    "ENABLE_FAST_MOBILE_SCROLLING=1",
     "ENABLE_MEDIA_CAPTURE=1",
   ]
 } else {
   feature_defines_list += [
     "ENABLE_INPUT_MULTIPLE_FIELDS_UI=1",
     "ENABLE_MEDIA_CAPTURE=0",
     "ENABLE_WEB_AUDIO=1",
   ]
 }
 
@@ skipping 23 matching lines @@
 }
 
 # feature_defines_string -------------------------------------------------------
 
 # Convert the list to a space-separated string for passing to scripts.
 # This would be the equivalent of passing '<(feature_defines)' in GYP.
 feature_defines_string = exec_script(
     "build/gn_list_to_space_separated_string.py",
     feature_defines_list,
     "trim string")