Fix bug where the tooltip would not be erased if it had been non-empty and then was updated to be empty.
Fix bug where the tooltip would not be erased if it had been non-empty and then was updated to be empty.

BUG=none
TEST=Start Chrome, type a letter in the omnibox, hover the Go button and wait for the tooltip.  Then delete the character and move the mouse pointer a tiny bit.  The tooltip should disappear.
Review URL: http://codereview.chromium.org/175032

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@24918 0039d316-1c4b-4281-b951-d872f2087c98

ropik/chromium,adobe/chromium,adobe/chromium,gavinp/chromium,adobe/chromium,Crystalnix/house-of-life-chromium,adobe/chromium,Crystalnix/house-of-life-chromium,Crystalnix/house-of-life-chromium,Crystalnix/house-of-life-chromium,Crystalnix/house-of-life-chromium,yitian134/chromium,ropik/chromium,adobe/chromium,yitian134/chromium,gavinp/chromium,gavinp/chromium,gavinp/chromium,ropik/chromium,adobe/chromium,yitian134/chromium,adobe/chromium,yitian134/chromium,Crystalnix/house-of-life-chromium,yitian134/chromium,yitian134/chromium,Crystalnix/house-of-life-chromium,Crystalnix/house-of-life-chromium,gavinp/chromium,ropik/chromium,adobe/chromium,gavinp/chromium,ropik/chromium,ropik/chromium,adobe/chromium,gavinp/chromium,Crystalnix/house-of-life-chromium,yitian134/chromium,adobe/chromium,yitian134/chromium,Crystalnix/house-of-life-chromium,yitian134/chromium,gavinp/chromium,ropik/chromium,ropik/chromium,ropik/chromium,gavinp/chromium,yitian134/chromium,Crystalnix/house-of-life-chromium,gavinp/chromium,adobe/chromium