[M146] Increment WebGL context generation number on context restore

Objects created while the context is lost should not be valid to use
after the context is restored.

• Replace number_of_context_losses_ with a "context generation number"
  which increments on both context loss and context restore.
  • Technically, it would make sense to increment it only on context
    restore, but just in case any logic is relying on the current
    behavior, increment it in both places.
  • It's uint64_t just in case someone figures out how to increment it 4
    billion times.
• Remove unused WebGLRenderingContextBase::number_of_context_losses_,
  left over from before it was moved into WebGLContextObjectSupport.

(cherry picked from commit c1433740f3ea902fd6b15d63c4865ad60a3761f9)

Bug: 485935305
Change-Id: I1007217c8e69cfb8de4f117e0b7845ca574579c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7630664
Reviewed-by: Kenneth Russell kbr@chromium.org
Commit-Queue: Kai Ninomiya kainino@chromium.org
Cr-Original-Commit-Position: refs/heads/main@{#1593726}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7658823
Auto-Submit: Kai Ninomiya kainino@chromium.org
Bot-Commit: Rubber Stamper rubber-stamper@appspot.gserviceaccount.com
Commit-Queue: Rubber Stamper rubber-stamper@appspot.gserviceaccount.com
Cr-Commit-Position: refs/branch-heads/7680@{#2370}
Cr-Branched-From: 76b7d80e5cda23fe6537eed26d68c92e995c7f39-refs/heads/main@{#1582197}

Notes: Backported fix for 485935305.