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A test window is just a path to write an image file to. Each window has
a unique image path.

BUG=392280
TEST=built with chromeos==1 use_ozone==1 & ran test platform
NOTRY=true

depends on https://codereview.chromium.org/375053002/

Review URL: https://codereview.chromium.org/377973005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@283875 0039d316-1c4b-4281-b951-d872f2087c98