Repeat window snapshot copy request on failure

We're hitting a bug where copy requests sporadically fail. The complete
fix is pretty involved (see https://crbug.com/644992 ), but just
repeating the copy request a few times should greatly reduce the
frequency of the problem and should be mergeable to M54.

BUG=644569

Review-Url: https://codereview.chromium.org/2349973009
Cr-Commit-Position: refs/heads/master@{#422012}
(cherry picked from commit cebea890c1e2a49072e0e5ec2bd8823ae2bb2a86)

Review URL: https://codereview.chromium.org/2390043002 .

Cr-Commit-Position: refs/branch-heads/2840@{#629}
Cr-Branched-From: 1ae106dbab4bddd85132d5b75c670794311f4c57-refs/heads/master@{#414607}
1 file changed