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The position returned from last_mouse_position() is in screen
co-ordinates but the calculation steps for the composition expect that
the cursor position be relative to the desktop dimentions.

This change transforms the screen relative position into a position
relative to the desktop window and hence relative to the secondary
display.

BUG=371761
TEST=Under ChromeOS capture a secondary screen and observe that the
pointer is now composed into the video steam in the correct place

Review URL: https://codereview.chromium.org/278873002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@269650 0039d316-1c4b-4281-b951-d872f2087c98