Add a comment to caution that timer::stop() should not be called too much

third_party/WebKit/Source/platform/Timer.h

Index: third_party/WebKit/Source/platform/Timer.h
diff --git a/third_party/WebKit/Source/platform/Timer.h b/third_party/WebKit/Source/platform/Timer.h
index 7027d26f4f5f79e11cc2aef637d7329f45c28788..295d0513c5d12a02c8a9a7d9d2497d787e2e68d0 100644
--- a/third_party/WebKit/Source/platform/Timer.h
+++ b/third_party/WebKit/Source/platform/Timer.h
@@ -58,6 +58,8 @@ public:
         start(interval, 0, caller);
     }
 
+    // Timer cancellation is supported but not free. Please be careful not to
+    // cause a flood of timer cancellations.
     void stop();
     bool isActive() const;
     const WebTraceLocation& location() const { return m_location; }