gpu: Hide service_utils.h/cc command line parsing functions

1) Hide ParseWebGPUAdapterName/PowerPreference since it's only used in
   service_utils.cc.

2) Fix GpuDataManagerImplPrivate to use features::IsUsingVulkan instead
   of making up its own Vulkan enabling logic. ParseVulkanImplementation
   becomes private to service_utils.cc as a result.

3) Add/fix some comments in GpuDataManagerImplPrivate and remove an
   unnecessary BUILDFLAG(ENABLE_VULKAN) define guard.

Bug: 345271828
Change-Id: I3416ddf0566165d9ce3df416ca25d52e61a18756
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5709761
Auto-Submit: Sunny Sachanandani <sunnyps@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1329156}