Export default implementation of getting wall-clock time.

Derived projects need easy access to the original V8's implementation of
time to implement Platform interface.

Bug: chromium:751993
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I97ee77929fda5930e7d75ca8609797673485cec3
Reviewed-on: https://chromium-review.googlesource.com/636884Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Sergei Datsenko <dats@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47638}

include/v8-platform.h

@@ -226,6 +226,14 @@ class Platform {
    * Returns an instance of a v8::TracingController. This must be non-nullptr.
    */
   virtual TracingController* GetTracingController() = 0;
+
+ protected:
+  /**
+   * Default implementation of current wall-clock time in milliseconds
+   * since epoch. Useful for implementing |CurrentClockTimeMillis| if
+   * nothing special needed.
+   */
+  static double SystemClockTimeMillis();
 };
 
 }  // namespace v8

src/v8.cc

@@ -130,4 +130,9 @@ void V8::SetSnapshotBlob(StartupData* snapshot_blob) {
 #endif
 }
 }  // namespace internal
+
+// static
+double Platform::SystemClockTimeMillis() {
+  return base::OS::TimeCurrentMillis();
+}
 }  // namespace v8