The CL includes below changes:
1. Change bool isAssociatedImageValid to void verifyAssociatedImageValid
since we always required that the validation check should never be false.
So ASSERT() is enough. Same to
Image11::isAssociatedStorageValid->Image11::verifyAssociatedStorageValid.
2. Remove the unnecessary if checking after ASSERT
3. Use override instead of virtual if the function is virtual and
is overriding a virtual function from the base class.

BUG=angleproject:2006

Change-Id: I036666ae1ed4bfcaa8cef9e0e9626d375cd81a27
Reviewed-on: https://chromium-review.googlesource.com/480015Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>