Identify viewports that trigger GPU rasterization

This adds a hasViewportThatTriggersGpuRasterization
method to WebView. The return value of this method
depends on the presence of the meta tag
<meta name="viewport" content="width=device-width, minimum-scale=1.0, minimal-ui">

BUG=367202

Review URL: https://codereview.chromium.org/259683002

git-svn-id: svn://svn.chromium.org/blink/trunk@172788 bbb929c8-8fbe-4397-9dbb-9b2b20218538