Add Mojo IPC for seeding new Renderer with Browser's cached blob state.

blimp/common/blob_cache/in_memory_blob_cache.cc

Index: blimp/common/blob_cache/in_memory_blob_cache.cc
diff --git a/blimp/common/blob_cache/in_memory_blob_cache.cc b/blimp/common/blob_cache/in_memory_blob_cache.cc
index bad46df059937c7849f89da6a897de8e93114612..55718ba59a39cc38efb86006f8cff939cd95dc3f 100644
--- a/blimp/common/blob_cache/in_memory_blob_cache.cc
+++ b/blimp/common/blob_cache/in_memory_blob_cache.cc
@@ -12,6 +12,14 @@ InMemoryBlobCache::InMemoryBlobCache() {}
 
 InMemoryBlobCache::~InMemoryBlobCache() {}
 
+std::vector<BlobId> InMemoryBlobCache::GetCachedBlobIds() const {
+  std::vector<BlobId> output;
+  for (const auto& pair : cache_) {
+    output.push_back(pair.first);
+  }
+  return output;
+}
+
 void InMemoryBlobCache::Put(const BlobId& id, BlobDataPtr data) {
   if (Contains(id)) {
     // In cases where the engine has miscalculated what is already available