gtk: Subtract the tabstrip x offset from the last tab x offset when calculating the available tab width.  The tabstrip and its tabs are not necessarily based at (0,0).
gtk: Subtract the tabstrip x offset from the last tab x offset when calculating the available tab width.  The tabstrip and its tabs are not necessarily based at (0,0).

BUG=20329
TEST=Open many tabs until they resize smaller.  Repeatedly close the left-most tab with the close button.  The tabs should not resize at all until the mouse leaves the tabstrip.
Review URL: http://codereview.chromium.org/173559

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@24630 0039d316-1c4b-4281-b951-d872f2087c98

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