Linux: show tab context menu on mousedown rather than mouse up.
Linux: show tab context menu on mousedown rather than mouse up.

TEST=right click on a tab and see a context menu popup (i.e. no regression). Also, right click, hold, drag, release to select an item.

Review URL: http://codereview.chromium.org/118023

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@17218 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser