Zero initialize the serial in dawn::wire::client::RequestTracker

Caught by MSAN  on Webkit Linux MSAN:
https://ci.chromium.org/ui/p/chromium/builders/ci/WebKit%20Linux%20MSAN/14833/overview

Bug: chromium:1315260
Change-Id: Ie3e07f7ef7a0c9fa77199c7058e87a7fd312b58e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/90462
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Loko Kung <lokokung@google.com>
Commit-Queue: Austin Eng <enga@chromium.org>
Commit-Queue: Loko Kung <lokokung@google.com>
Auto-Submit: Austin Eng <enga@chromium.org>

src/dawn/wire/client/RequestTracker.h

@@ -72,7 +72,7 @@ class RequestTracker : NonCopyable {
     }
 
   private:
-    uint64_t mSerial;
+    uint64_t mSerial = 0;
     std::map<uint64_t, Request> mRequests;
 };