Update image content rect on src changes even if intrinsic size matches

This patch changes RenderImage::imageDimensionsChanged to update the
image content rect even if the image's intrinsic size is equal. This
fixes a bug where changing an SVG image to another image of the same
size would not correctly update the image content rect.

This patch also cleans up a confusing variable name in
RenderImage::updateInnerContentRect, changing paint rect to container
rect to better match what it is.

BUG=269946

Review URL: https://chromiumcodereview.appspot.com/23890043

git-svn-id: svn://svn.chromium.org/blink/trunk@158343 bbb929c8-8fbe-4397-9dbb-9b2b20218538