Remove unnecessary comment in RenderImage::imageDimensionsChanged

I asked mstensho@opera.com to add this FIXME, but in the comments
in crbug.com/275755 it became clear that I didn't understand the code.
This patch removes the incorrect FIXME.

NOTRY=true
TBR=mstensho@opera.com
BUG=275755

Review URL: https://chromiumcodereview.appspot.com/23478018

git-svn-id: svn://svn.chromium.org/blink/trunk@157044 bbb929c8-8fbe-4397-9dbb-9b2b20218538