Crash fix.
Crash fix.

We never reset |tab_contents_| to NULL anywhere (and indeed we can't,
even in ReleasePreviewContents(), because browser.cc accesses it). We
were protected from acccessing a dangling tab_contents_ before, due to
the is_active() check that preceded a call to PrepareForCommit(). That
was removed in http://crrev.com/105664 however; hence this fix.

BUG=100521
TEST=none

Review URL: http://codereview.chromium.org/8329020

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@106075 0039d316-1c4b-4281-b951-d872f2087c98

yitian134/chromium,ropik/chromium,adobe/chromium,adobe/chromium,adobe/chromium,ropik/chromium,adobe/chromium,yitian134/chromium,gavinp/chromium,yitian134/chromium,ropik/chromium,ropik/chromium,gavinp/chromium,adobe/chromium,gavinp/chromium,yitian134/chromium,ropik/chromium,gavinp/chromium,ropik/chromium,adobe/chromium,gavinp/chromium,yitian134/chromium,gavinp/chromium,gavinp/chromium,ropik/chromium,ropik/chromium,adobe/chromium,gavinp/chromium,yitian134/chromium,adobe/chromium,adobe/chromium,ropik/chromium,adobe/chromium,yitian134/chromium,gavinp/chromium,gavinp/chromium,yitian134/chromium,adobe/chromium,yitian134/chromium,yitian134/chromium