Normalize storage of shader strings in the cache

Adds a tag that can be used to verify the expected type
of shaders. My follow-up CL that adds SkSL editing to
Vulkan relies heavily on this.

Change-Id: Ifda420c2dcbaff07cdf1b8157d0ece02b1ab6c78
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/210262
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
8 files changed