Support --shader-cache-path for loading shader caches.

--shader-cache-path was only used to direct Chrome where to save shader
caches when generating them. Now it can be used for telling Chrome
where to load it too.

Fix that this flag was not forwarded to the GPU process.

Bug: chromium:1481238
Change-Id: I467f434d3a1a8f4fbbd7fefb6a337f093fda3044
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4863878
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1202884}