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The coded size should always be rounded up to multiples of 16 pixels to accomodate
alignment requirements of HW encoders, while the visible size reflects the actual
content size.

Update video capture messages to pass visible rect from browser to renderer process.

BUG=402501
TEST=verify that non-multiple-of-16-pixel capture works and no black border is shown
R=dalecurtis@chromium.org, hclam@chromium.org, palmer@chromium.org

Review URL: https://codereview.chromium.org/500653003

Cr-Commit-Position: refs/heads/master@{#292717}