ccpr: Don't use the GrContext id for the path cache id

Currently this is probably safe, but somebody could easily make a
change in the future that adds another path cache to the context and
accidentally introduces tricky bugs.

TBR=brianosman@google.com

Bug: chromium:897510
Bug: chromium:897413
Bug: chromium:897245
Bug: chromium:897507
Change-Id: I6bc40ac671058f78eb290dd775612d99008d32e7
Reviewed-on: https://skia-review.googlesource.com/c/164700
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
9 files changed