[date] Add a cache for timezone names to DateCache

src/date.h

Index: src/date.h
diff --git a/src/date.h b/src/date.h
index 0a3e91beb26589e1b5078e420bfe07a8b18ada25..9f081bb185593e02b5d9e543172f990b4f0d842b 100644
--- a/src/date.h
+++ b/src/date.h
@@ -93,7 +93,12 @@ class DateCache {
     if (time_ms < 0 || time_ms > kMaxEpochTimeInMs) {
       time_ms = EquivalentTime(time_ms);
     }
-    return base::OS::LocalTimezone(static_cast<double>(time_ms), tz_cache_);
+    bool is_dst = DaylightSavingsOffsetInMs(time_ms) != 0;
+    const char** name = is_dst ? &dst_tz_name_ : &tz_name_;

[jgruber, 2017/03/03 08:09:57]

Nit: Can you use plain 'const char* name' instead of 'const char**'?

[Dan Ehrenberg, 2017/03/03 12:57:44]

I don't think so, because then I don't see how I could write to it on line 99
and have that affect the underlying object. I could use a C++ reference, but
non-const references are discouraged in the Google style guide (though I believe
they're found all over Blink).

[jgruber, 2017/03/03 13:02:34]

Ah, gotcha. sgtm.

+    if (*name == nullptr) {
+      *name = base::OS::LocalTimezone(static_cast<double>(time_ms), tz_cache_);
+    }
+    return *name;
   }
 
   // ECMA 262 - 15.9.5.26
@@ -277,6 +282,10 @@ class DateCache {
   int ymd_month_;
   int ymd_day_;
 
+  // Timezone name cache
+  const char* tz_name_;
+  const char* dst_tz_name_;
+
   base::TimezoneCache* tz_cache_;
 };