Remove stale comment

cachedImage->imageForLayoutObject() no longer returns
BitmapImages. The surrounding code is still correct since
cachedImage->imageForLayoutObject() may return a SVGImageForContainer
which does not propagate the correct filename extension.

NOTRY=true
BUG=128055

Review URL: https://codereview.chromium.org/1429803002

Cr-Commit-Position: refs/heads/master@{#357105}
1 file changed