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This CL fixes the regression that Combobox doesn't get focused when
clicking a right button of a mouse. Before introducing
TransparentButton, Combobox can get focus by itself on the mouse button
events, but now, TranparentButton robs the combobox of the events.
This CL has TransparentButton::OnMousePressed help the combobox with
focusing it.

BUG=350811
TEST=manual

Review URL: https://codereview.chromium.org/189913014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256016 0039d316-1c4b-4281-b951-d872f2087c98