Revert of Implement enableFastMobileScrolling web api. (https://codereview.chromium.org/321473002/)

Reason for revert:
Broke the build.

Original issue's description:
> Implement enableFastMobileScrolling web api.
> 
> 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 1st set of changes.
> 
> BUG=381391
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=175621

TBR=tkent@chromium.org,adamk@chromium.org,c.shu@samsung.com
NOTREECHECKS=true
NOTRY=true
BUG=381391

Review URL: https://codereview.chromium.org/317313002

git-svn-id: svn://svn.chromium.org/blink/trunk@175622 bbb929c8-8fbe-4397-9dbb-9b2b20218538