Add Platform::isLowEndDeviceMode()
Add Platform::isLowEndDeviceMode()

So that Blink can take actions to reduce memory usage on low end
devices.
(Chromium side CL: https://codereview.chromium.org/1287813002)

BUG=519146

Review URL: https://codereview.chromium.org/1286873002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@200366 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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