chromeos: Migrate ChromeScreenshotGrabber to TaskScheduler

Use the same sequenced task runner for ChromeScreenshotGrabber and
ui::ScreenshotGrabber to ensure that multiple screenshots taken
rapidly are processed in the same order.

Bug: 667892, 737162
Test: manually take screenshots with Ctrl-F5, check disk and clipboard
Change-Id: I48f557adb6dc9f7b63ab90c852947f6505e344de
Reviewed-on: https://chromium-review.googlesource.com/549970
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#484181}
1 file changed