Fix browser crash when tabs that call chrome.webstore.install get closed before the call completes

This manifested itself as WebstoreInlineInstallTest.ArgumentValidation test
crashes because the test would make several chrome.webstore.install() calls
and not wait for responses before the test completed.

The fix was to make WebstoreInlineInstaller subclass TabContentsObserver and
check if the tab is still alive before trying to send responses back to it.

BUG=95713

Review URL: http://codereview.chromium.org/7861008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@100295 0039d316-1c4b-4281-b951-d872f2087c98
4 files changed