[platform] Return task runners as shared_ptr

At the moment, task runners are returned as unique_ptr. This is
inconvenient, however. In all implementations I did, the platform holds
a shared pointer of the task runner and wraps it in a wrapper class just
to return it as a unique_ptr. With this CL the platform API is changed
to return a shared_ptr directly.

R=rmcilroy@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ide278db855199ea239ad0ae14d97fd17349dac8c
Reviewed-on: https://chromium-review.googlesource.com/768867
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49366}

include/v8-platform.h

@@ -199,7 +199,7 @@ class Platform {
    * Returns a TaskRunner which can be used to post a task on the foreground.
    * This function should only be called from a foreground thread.
    */
-  virtual std::unique_ptr<v8::TaskRunner> GetForegroundTaskRunner(
+  virtual std::shared_ptr<v8::TaskRunner> GetForegroundTaskRunner(
       Isolate* isolate) {
     // TODO(ahaas): Make this function abstract after it got implemented on all
     // platforms.
@@ -210,7 +210,7 @@ class Platform {
    * Returns a TaskRunner which can be used to post a task on a background.
    * This function should only be called from a foreground thread.
    */
-  virtual std::unique_ptr<v8::TaskRunner> GetBackgroundTaskRunner(
+  virtual std::shared_ptr<v8::TaskRunner> GetBackgroundTaskRunner(
       Isolate* isolate) {
     // TODO(ahaas): Make this function abstract after it got implemented on all
     // platforms.