Add comment on StyleInvalidator trace events

Warn that StyleInvalidator methods are performance sensitive and
add notes about its implementation.

TBR=pdr,haraken
NOTRY=true
only comment additions. confirmed compile pass.
BUG=410701

Review URL: https://codereview.chromium.org/650343004

git-svn-id: svn://svn.chromium.org/blink/trunk@183961 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/css/invalidation/StyleInvalidator.cpp

@@ -18,6 +18,12 @@
 
 namespace blink {
 
+// StyleInvalidator methods are super sensitive to performance benchmarks.
+// We easily get 1% regression per additional if statement on recursive
+// invalidate methods.
+// To minimize performance impact, we wrap trace events with a lookup of
+// cached flag. The cached flag is made "static const" and is not shared
+// with DescendantInvalidationSet to avoid additional GOT lookup cost.
 static const unsigned char* s_tracingEnabled = nullptr;
 
 #define TRACE_STYLE_INVALIDATOR_INVALIDATION_IF_ENABLED(element, reason) \