[android_webview] Set 'fling active' flag only from computeScroll.

This prevents us getting stuck in smoothness mode if the embedding application
inhibits the View.computeScroll callback.

BUG=339245

Review URL: https://codereview.chromium.org/149963002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247930 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247930