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Previously OnCaptureFailure could be called just after CaptureAsync call
 in case, when view actually was absent. That led to 2 calls of
 SendResponse. Extension functions are designed to receive 1 response
 on each call, so 2 calls is bad behavior.
Now web capture clients behavior fixed.

R=fsamuel@chromium.org, tbarzic@chromium.org

Change-Id: I2ce195aaa3698d9c38ca2fa0e3735e6f9555f873
Reviewed-on: https://chromium-review.googlesource.com/930762
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539994}