cache SkSL headers

This reduces the cost of successive shader compilations by caching the
results of compiling SkSL's headers.

Bug: skia:
Change-Id: If7fc21a9877021c4025ad99dd0981523a25855e0
Reviewed-on: https://skia-review.googlesource.com/123422
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
12 files changed