[Android] Fix flaky ContentViewGestureHandlerTest

For |testDoubleTapDragZoomCancelledOnSecondaryPointerDown()|, we were using
ACTION_POINTER_{UP,DOWN} with MotionEvents that had but one pointer. This was
confusing the ScaleGestureDetector, now fixed by properly synthesizing
MotionEvents with multiple pointer coordinates.

BUG=339943
NOTRY=true

Review URL: https://codereview.chromium.org/134473006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248324 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248324