Do not start 'dragging' of partial screenshot when mouse is pressed.
Do not start 'dragging' of partial screenshot when mouse is pressed.

In case that mouse is just pressed and released immediately, it should
not take screenshot.

BUG=120475
TEST=manually

Review URL: https://chromiumcodereview.appspot.com/10006001

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@131100 0039d316-1c4b-4281-b951-d872f2087c98

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