InMemoryVideoDecodeStatsDBImpl: Check for re-init

Small number of crashes in the linked bug. Re-init seems the only
possible explanation. Not obvious how that might occur.

Bug: 1114128
Change-Id: Idf34a1b7bdf11f43cf6469f9d5b24ce10ec79e7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2342390
Commit-Queue: Chrome Cunningham <chcunningham@chromium.org>
Commit-Queue: Frank Liberato <liberato@chromium.org>
Auto-Submit: Chrome Cunningham <chcunningham@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796188}

media/capabilities/in_memory_video_decode_stats_db_impl.cc

@@ -39,6 +39,11 @@ void InMemoryVideoDecodeStatsDBImpl::Initialize(InitializeCB init_cb) {
   DCHECK(init_cb);
   DCHECK(!db_init_);
 
+  // Tracking down crbug.com/1114128. Suspect we're double initializing somehow,
+  // so this should show who the crash at the time of the second initialize
+  // call.
+  CHECK(!seed_db_) << __func__ << " Already have a seed_db_?";
+
   // Fetch an *initialized* seed DB.
   if (seed_db_provider_) {
     seed_db_provider_->GetVideoDecodeStatsDB(