Don't load all bytes of video when generating thumbnails.
Don't load all bytes of video when generating thumbnails.

Previously ImageLoader loaded all bytes of video by using XHR before
generating thumbnails. The CL fixes it so that ImageLoader assign entry
URL to video tag directly, which avoids from loading all bytes to
memory.

BUG=597243
TEST=manually

Review URL: https://codereview.chromium.org/1842313005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#384515}

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