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Accumulating large draw command backlogs can result in janky
performance, and in extreme cases, a GPU denial of service. This
change fixes that problem by disabling deferred rendering when
high overdraw is detected. Cases where the overdraw overwrites
the entire canvas are not affected by this change and will
stay in deferred rendering mode because overwriting the entire
canvas resets the backlog.

BUG=522764, 522972

Review URL: https://codereview.chromium.org/1308853003

git-svn-id: svn://svn.chromium.org/blink/trunk@201036 bbb929c8-8fbe-4397-9dbb-9b2b20218538