Bug 585152 - Reland Date.now() fractional changes, along with fixed tests. (r=jwalden,dolske,gavin, rs=jwalden)

For the tests, I put a Math.floor around anything which looked like it did string concatenation. In principal that was r+ by dolske and gavin, and jwalden r+ed the extension to the rest of the tests.

Backout policy: the jsdate.cpp changes can be backed out trivially. The test changes should backout cleanly, but we're not hosed if they don't.

js/src/jsdate.cpp

@@ -1189,10 +1189,16 @@ NowAsMillis()
     return (jsdouble) (PRMJ_Now() / PRMJ_USEC_PER_MSEC);
 }
 
+static inline jsdouble
+NowAsFractionalsMillis()
+{
+    return (jsdouble) (PRMJ_Now() / double(PRMJ_USEC_PER_MSEC));
+}
+
 static JSBool
 date_now(JSContext *cx, uintN argc, Value *vp)
 {
-    vp->setDouble(NowAsMillis());
+    vp->setDouble(NowAsFractionalsMillis());
     return JS_TRUE;
 }
 
@@ -1200,7 +1206,7 @@ date_now(JSContext *cx, uintN argc, Value *vp)
 static jsdouble FASTCALL
 date_now_tn(JSContext*)
 {
-    return NowAsMillis();
+    return NowAsFractionalsMillis();
 }
 #endif