Remove unnamed 'ExceptionState&' parameters.

Two functions in 'Source/core/html/canvas/WebGLRenderingContext.cpp'
accept 'ExceptionState' parameters, but don't name or use them. This
patch removes those parameters, and adjusts our IDL's claims about
WebGLRenderingContext::readPixels accordingly.

BUG=240693

Review URL: https://chromiumcodereview.appspot.com/21929003

git-svn-id: svn://svn.chromium.org/blink/trunk@155586 bbb929c8-8fbe-4397-9dbb-9b2b20218538