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

Review URL: https://codereview.chromium.org/321473002

git-svn-id: svn://svn.chromium.org/blink/trunk@175621 bbb929c8-8fbe-4397-9dbb-9b2b20218538