Add removeUserDefinedSnippet method to ShaderCodeDictionary.

We need to purge out user snippets when they are no longer needed, to
avoid unbounded memory usage.

Change-Id: Ib33909ec8c94cd6272f1a28e52a7ab92b27dfb0d
Bug: skia:13405
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/552577
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
3 files changed