Add gltestpersistentcache config that tests GrContextOption's cache.

Uses a new GPU sink that runs each test twice, once to populate the
cache and then again with a new GrContext but a warmed cache. It
verifies that the two generated images are the same.

Change-Id: Iaba195a69751f14ea946afe7174228a813b83a63
Reviewed-on: https://skia-review.googlesource.com/140567
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 11b1943..b7a5c04 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1239,6 +1239,8 @@
     sources = [
       "tools/gpu/GrContextFactory.cpp",
       "tools/gpu/GrTest.cpp",
+      "tools/gpu/MemoryCache.cpp",
+      "tools/gpu/MemoryCache.h",
       "tools/gpu/ProxyUtils.cpp",
       "tools/gpu/TestContext.cpp",
       "tools/gpu/atlastext/GLTestAtlasTextRenderer.cpp",