Add dawn_platform::Platform for handling tracing events in Dawn

This patch provides Chromium TRACE_EVENT macros and hooks for
implementing the TRACE_EVENT api.

Bug: chromium:958013
Change-Id: I033b1c7ca57c550504a1bea1898a1a152831922b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/7060
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

src/dawn_native/Instance.cpp

@@ -192,4 +192,12 @@ namespace dawn_native {
         return mBeginCaptureOnStartup;
     }
 
+    void InstanceBase::SetPlatform(dawn_platform::Platform* platform) {
+        mPlatform = platform;
+    }
+
+    dawn_platform::Platform* InstanceBase::GetPlatform() const {
+        return mPlatform;
+    }
+
 }  // namespace dawn_native