WebGL: remove redundant contentChanged callback when resizing.

HTMLCanvasElement::reset() already notifies it, so
WebGLRenderingContextBase::reshape() doesn't need to do it.

Review URL: https://codereview.chromium.org/666393002

git-svn-id: svn://svn.chromium.org/blink/trunk@184619 bbb929c8-8fbe-4397-9dbb-9b2b20218538