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In order to avoid an ugly flash of white pixels when a new app window is
shown, this patch defers showing a window until a non-empty backing
store update has completed.

The feature is behind the "enable-apps-show-on-first-paint" flag.

BUG=225843, 254152

Review URL: https://codereview.chromium.org/55293003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233422 0039d316-1c4b-4281-b951-d872f2087c98