document.lastModified should consider user's local time zone

Source/wtf/DateMath.cpp

Index: Source/wtf/DateMath.cpp
diff --git a/Source/wtf/DateMath.cpp b/Source/wtf/DateMath.cpp
index 1c901b712d3aee23be9ea78c6c68e416efcab2d2..1c585c090382e7536ffc9393607c2947abe209ac 100644
--- a/Source/wtf/DateMath.cpp
+++ b/Source/wtf/DateMath.cpp
@@ -358,7 +358,7 @@ static int equivalentYearForDST(int year)
     return year;
 }
 
-int32_t calculateUTCOffset()
+static int32_t calculateUTCOffset()
 {
 #if OS(WIN)
     TIME_ZONE_INFORMATION timeZoneInformation;
@@ -431,7 +431,7 @@ static double calculateDSTOffsetSimple(double localTimeSeconds, double utcOffset
 }
 
 // Get the DST offset, given a time in UTC
-double calculateDSTOffset(double ms, double utcOffset)
+static double calculateDSTOffset(double ms, double utcOffset)
 {
     // On Mac OS X, the call to localtime (see calculateDSTOffsetSimple) will return historically accurate
     // DST information (e.g. New Zealand did not have DST from 1946 to 1974) however the JavaScript
@@ -881,4 +881,11 @@ String makeRFC2822DateString(unsigned dayOfWeek, unsigned day, unsigned month, u
     return stringBuilder.toString();
 }
 
+double convertToLocalTime(double ms)
+{
+    double utcOffset = calculateUTCOffset();
+    double dstOffset = calculateDSTOffset(ms, utcOffset);
+    return (ms + utcOffset + dstOffset);
+}
+
 } // namespace WTF