For performance reason, we choose to block the picture

update during drag and fling. For sites like nytimes.com,
there is a JavaScript constantly updating the content.
If we don't block the live update during drag/fling,
frame rate can drop by 30%.

This essentially reverted the CL for http://b/issue?id=2310295.
But as we honor preventDefault on touch move now,
the two sites mentioned in that bug do not have problem
even after this CL.

Fix http://b/issue?id=2574172