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This CL implements ImageCapture's takePhoto() method by enqueueing a
callback every time the method is called. Check the queue when a new
frame is captured in DoCapture(), and execute callbacks with a blob
argument.

Add a new method GetPhotoBlob() to construct blob. If the source image
is originally in JPEG format, it's copied to blob directly. Otherwise
convert the source image to RGB format and encode it to PNG.

R=mcasas@chromium.org
BUG=646430
TEST=All tests in VideoCaptureDeviceTests passed.

Review-Url: https://codereview.chromium.org/2344123002
Cr-Commit-Position: refs/heads/master@{#419484}