Avoid using ToInternalValue in computing timings, so that the timings are...

Avoid using ToInternalValue in computing timings, so that the timings are consistent accross platforms and with javascript performance timings API.

BUG=413672

Review URL: https://codereview.chromium.org/557953006

Cr-Commit-Position: refs/heads/master@{#295123}

components/startup_metric_utils/startup_metric_utils.cc

@@ -47,8 +47,7 @@ void RecordMainEntryTimeHistogram() {
   const int kLowWordMask = 0xFFFFFFFF;
   const int kLower31BitsMask = 0x7FFFFFFF;
   base::TimeDelta browser_main_entry_time_absolute =
-      base::TimeDelta::FromMilliseconds(
-          MainEntryPointTimeInternal()->ToInternalValue() / 1000.0);
+      *MainEntryPointTimeInternal() - base::Time::UnixEpoch();
 
   uint64 browser_main_entry_time_raw_ms =
       browser_main_entry_time_absolute.InMilliseconds();

content/renderer/stats_collection_controller.cc

@@ -47,7 +47,7 @@ bool CurrentRenderViewImpl(RenderViewImpl** out) {
 // Example return value:
 // {'load_start_ms': 1, 'load_duration_ms': 2.5}
 // either value may be null if a web contents hasn't fully loaded.
-// load_start_ms is represented as milliseconds since system boot.
+// load_start_ms is represented as milliseconds since the unix epoch.
 void ConvertLoadTimeToJSON(
     const base::Time& load_start_time,
     const base::Time& load_stop_time,
@@ -57,7 +57,8 @@ void ConvertLoadTimeToJSON(
   if (load_start_time.is_null()) {
     item.Set("load_start_ms", base::Value::CreateNullValue());
   } else {
-    item.SetDouble("load_start_ms", load_start_time.ToInternalValue() / 1000);
+    item.SetDouble("load_start_ms", (load_start_time - base::Time::UnixEpoch())
+                   .InMillisecondsF());
   }
   if (load_start_time.is_null() || load_stop_time.is_null()) {
     item.Set("load_duration_ms", base::Value::CreateNullValue());