Avoid CHECK() failure when painting avatar menu button

Avoid CHECK() failure when painting avatar menu button

Following the refactoring in https://codereview.chromium.org/1009403002
the Linux ChromiumOS Tests (dbg) browser_tests failed 22/25 times since
this patch landed:
http://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Tests%20%28dbg%29%281%29/builds/2094/

It looks like this was due to the empty gfx::Image being passed in to
GetAvatarIconForTitleBar() being returned. It is not allowed to call
ToImageSkia() on an empty gfx::Image. The alternative method
AsImageSkia() will return an empty gfx::ImageSkia if the gfx::Image is
empty.

TBR=yiyaoliu@chromium.org,msw@chromium.org
BUG=452524

[robert.bradford, 2015-05-12 12:08:51]

robert.bradford@intel.com changed reviewers:
+ yiyaoliu@chromium.org
- yao@chromium.org

[msw, 2015-05-12 16:55:01]

lgtm if the refactoring is relanded.

[yao, 2015-05-13 19:32:12]

Hi Robert, 

Thanks for working on this. Do you want me to combine this CL into my original
refactoring CL? I don't wanna submit it and break try bots again if this CL is
not landed soon enough after that. 

Thanks, 
Yiyao

[robert.bradford, 2015-05-14 10:59:37]

On 2015/05/13 19:32:12, yao wrote:
> Hi Robert, 
> 
> Thanks for working on this. Do you want me to combine this CL into my original
> refactoring CL? I don't wanna submit it and break try bots again if this CL is
> not landed soon enough after that. 
> 
> Thanks, 
> Yiyao

Yes, please roll it in if you think it's the right fix.