Fix debug failures in fileicon_source_cros
Fix debug failures in fileicon_source_cros

Remove a DCHECK that fires for unknown extensions, which are numerous.
Fix FilePath creation from an ascii absolute path.

BUG=NONE
TEST=Open chrome://downloads in debug mode and don't see DCHECKs fail.
Review URL: http://codereview.chromium.org/7027026

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@87508 0039d316-1c4b-4281-b951-d872f2087c98

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