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This patch pulls computed media query lengths values from backend.
Note: it is ok to create multiple MediaValuesDynamic objects because
they don't have any internal state and serve as a helper wrappers.

BUG=382996

Review URL: https://codereview.chromium.org/339553004

git-svn-id: svn://svn.chromium.org/blink/trunk@176445 bbb929c8-8fbe-4397-9dbb-9b2b20218538