Add UseCounter for simplifying loading transparent placeholder images

This CL introduces the following UseCounter:
  - SimplifyLoadingTransparentPlaceholderImage

This is useful to know how often we are able to use the simple path for
loading transparent 1x1 gifs.

Bug: 41496436
Change-Id: Id437dfc172b6c358d6cc15de7fd9fd341b44cf83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5420550


Reviewed-by: Sun Yueru <yrsun@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Nidhi Jaju <nidhijaju@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283529}