Fix crash when creating an ImageBitmap from an invalid canvas

BUG=354356

Review URL: https://codereview.chromium.org/211313003

git-svn-id: svn://svn.chromium.org/blink/trunk@169973 bbb929c8-8fbe-4397-9dbb-9b2b20218538
ScriptPromise ImageBitmapFactories::createImageBitmap(EventTarget& eventTarget, HTMLCanvasElement* canvas, ExceptionState& exceptionState)
{
    return createImageBitmap(eventTarget, canvas, 0, 0, canvas->width(), canvas->height(), exceptionState);
}

ScriptPromise ImageBitmapFactories::createImageBitmap(EventTarget& eventTarget, HTMLCanvasElement* canvas, ExceptionState& exceptionState)
{
    return createImageBitmap(eventTarget, canvas, 0, 0, canvas->width(), canvas->height(), exceptionState);
}
