Reland "Make GPU cache invalidation SkMessageBus messages go to one GrContext."

This is a reland of f4c5bb9aba485aa47c27b15905d81992b7cf4707

Original change's description:
> Make GPU cache invalidation SkMessageBus messages go to one GrContext.
> 
> Makes it so the template param to SkMessageBus must implement:
> bool shouldSend(uint32_t inboxID) const
> 
> Updates all GPU backend message types to only go to the GrContext that
> is adding a cache entry.
> 
> Bug: skia:
> Change-Id: I3e8a4eb90654b7b8ac57cac9fb508c0ef1d51058
> Reviewed-on: https://skia-review.googlesource.com/140220
> Reviewed-by: Robert Phillips <robertphillips@google.com>
> Reviewed-by: Jim Van Verth <jvanverth@google.com>

Bug: skia:
Change-Id: I8402bfe3ed0170c99936d47050458817030b473b
Reviewed-on: https://skia-review.googlesource.com/140801
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/tests/ResourceCacheTest.cpp b/tests/ResourceCacheTest.cpp
index dceb3e7..5b7a67a 100644
--- a/tests/ResourceCacheTest.cpp
+++ b/tests/ResourceCacheTest.cpp
@@ -1003,8 +1003,8 @@
     typedef SkMessageBus<GrUniqueKeyInvalidatedMessage> Bus;
 
     // Invalidate two of the three, they should be purged and no longer accessible via their keys.
-    Bus::Post(Msg(key1));
-    Bus::Post(Msg(key2));
+    Bus::Post(Msg(key1, context->uniqueID()));
+    Bus::Post(Msg(key2, context->uniqueID()));
     cache->purgeAsNeeded();
     // a should be deleted now, but we still have a ref on b.
     REPORTER_ASSERT(reporter, !cache->hasUniqueKey(key1));
@@ -1013,7 +1013,7 @@
     REPORTER_ASSERT(reporter, cache->hasUniqueKey(key3));
 
     // Invalidate the third.
-    Bus::Post(Msg(key3));
+    Bus::Post(Msg(key3, context->uniqueID()));
     cache->purgeAsNeeded();
     // we still have a ref on b, c should be recycled as scratch.
     REPORTER_ASSERT(reporter, 2 == TestResource::NumAlive());