Add get current wall-clock time to the Platform

To avoid breaking Chromium we expose a stub method first, and will start
using it only when V8 rolls into Chromium and we implement it there.

Bug: chromium:751993
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ida1f96f2dfa833552e7adfa36a580a6ef1bdd1aa
Reviewed-on: https://chromium-review.googlesource.com/604812
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Sergei Datsenko <dats@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47233}

include/v8-platform.h

@@ -204,6 +204,16 @@ class Platform {
    * the epoch.
    **/
   virtual double MonotonicallyIncreasingTime() = 0;
+
+  /**
+   * Current wall-clock time in milliseconds since epoch.
+   * This function is expected to return at least millisecond-precision values.
+   */
+  virtual double CurrentClockTimeMillis() {
+    // TODO(dats): Make pure virtual after V8 roll in Chromium.
+    return 0.0;
+  }
+
   typedef void (*StackTracePrinter)();
 
   /**