Fix frame viewer: LayerTypeAsString() can't be called from ~LayerImpl

For details on the fix see:
https://code.google.com/p/trace-viewer/source/detail?r=1165

R=enne@chromium.org

Review URL: https://codereview.chromium.org/179063003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253336 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253336