[GLImage] Remove OnMemoryDump() from cross-platform interface

The only remaining subclass of GLImage with a non-empty implementation
of GLImage::OnMemoryDump() is GLImageIOSurface. The one callsite of
that method on that class knows that it's talking to GLImageIOSurface.
This CL does the following:

1. Eliminates GLImage::OnMemoryDump() and all its empty implementations
2. Eliminates the code in Texture::DumpLevelMemory() that invokes
   GLImage::OnMemoryDump() as all subclasses that could go through that
   codepath have empty implementations [Texture is used with the
   validating decoder, which is no longer used on Mac]. Instead,
   changes this code to always dump the memory of the texture
   allocation manually.

Note that (2) might result in reported memory size increases. These are
not real changes but simply the result of more accurate tracking.

Bug: 1310018
Change-Id: Id337fd306c3cab5f2504e24554831b6f492f09a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4188603
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Reviewed-by: Peng Huang <penghuang@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1096239}