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Current Copy Image feature downloads the image outside WKWebView. This
CL adds functionality of fetching image's data by JavaScript in 2 steps:

1. Draws <img> to <canvas> and exports it;
2. If 1 failed, downloads the picture by XMLHttpRequest, which may be
  responsed from cache.

Both 2 methods may fail due to cross-origin restriction, but under
different conditions.

More info in this design doc:
https://docs.google.com/document/d/1twZ1-Ssn4_9L4eN23-7H-wJkKKWSWLJTSt6Kwn0DqsU/edit?usp=sharing

Bug: 163201
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I12bac9d6bd40b4c42d35a23745edaa4dca9fa071
Reviewed-on: https://chromium-review.googlesource.com/1145276
Commit-Queue: Yi Su <mrsuyi@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581176}