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/src/gpu/text/GrTextBlobCache.cpp b/src/gpu/text/GrTextBlobCache.cpp
index 71d9052..a60903f 100644
--- a/src/gpu/text/GrTextBlobCache.cpp
+++ b/src/gpu/text/GrTextBlobCache.cpp
@@ -30,7 +30,7 @@
 
 void GrTextBlobCache::PostPurgeBlobMessage(uint32_t blobID, uint32_t cacheID) {
     SkASSERT(blobID != SK_InvalidGenID);
-    SkMessageBus<PurgeBlobMessage>::Post(PurgeBlobMessage({blobID}), cacheID);
+    SkMessageBus<PurgeBlobMessage>::Post(PurgeBlobMessage(blobID, cacheID));
 }
 
 void GrTextBlobCache::purgeStaleBlobs() {
@@ -38,7 +38,7 @@
     fPurgeBlobInbox.poll(&msgs);
 
     for (const auto& msg : msgs) {
-        auto* idEntry = fBlobIDCache.find(msg.fID);
+        auto* idEntry = fBlobIDCache.find(msg.fBlobID);
         if (!idEntry) {
             // no cache entries for id
             continue;
@@ -51,7 +51,7 @@
         }
 
         // drop the idEntry itself (unrefs all blobs)
-        fBlobIDCache.remove(msg.fID);
+        fBlobIDCache.remove(msg.fBlobID);
     }
 }