Fix browser crash on startup.
Fix browser crash on startup.

>	chrome.dll!`anonymous namespace'::OnNoMemory()  Line 126	C++
	chrome.dll!_invalid_parameter_noinfo()  Line 99 + 0xc bytes	C++
	chrome.dll!RefCountedBytes::front()  Line 67 + 0x1b bytes	C++
	chrome.dll!BookmarkModel::OnFavIconDataAvailable(int handle=8, bool know_favicon=true, scoped_refptr<RefCountedBytes> data={...}, bool expired=true, GURL icon_url={...})  Line 652 + 0x2b bytes	C++

It seems RefCountedBytes data has no data. If data size is 0, accessing front
of data would cause crash.
By checking its size, fixes this crash.

BUG=25803
TEST=no crash on startup

Review URL: http://codereview.chromium.org/328037

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@30171 0039d316-1c4b-4281-b951-d872f2087c98

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