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: http://src.chromium.org/svn/trunk/src@24630 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: b08b42ab71b32507321f5c283d816f0cd7d0e3a8
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser