Bug 727695. Add TimeDuration::FromMicroseconds. r=cjones

xpcom/ds/TimeStamp.h

@@ -93,6 +93,9 @@ public:
     return FromMilliseconds(aSeconds * 1000.0);
   }
   static TimeDuration FromMilliseconds(double aMilliseconds);
+  static inline TimeDuration FromMicroseconds(double aMicroseconds) {
+    return FromMilliseconds(aMicroseconds / 1000.0);
+  }
 
   TimeDuration operator+(const TimeDuration& aOther) const {
     return TimeDuration::FromTicks(mValue + aOther.mValue);