Allow ImageBitmaps derived from HTMLImageElements to be evicted from RAM.

Since we were pre-cropping the HTMLImageElement before drawing, the 
CachedImage::didDraw(Image) command exited early because the Image argument
was different from the image referenced in the CachedImage. This has been fixed
by not pre-cropping the image and allowing drawBitmapRectToRect to handle the 
cropping.

TEST=manual
BUG=259947, 166658

Review URL: https://chromiumcodereview.appspot.com/22613002

git-svn-id: svn://svn.chromium.org/blink/trunk@155805 bbb929c8-8fbe-4397-9dbb-9b2b20218538