Use static_cast instead of reinterpret_cast for downcasting

static_cast is better suited for downcasting since it can check that the
types are related.

/components/gcm_driver
This CL was uploaded by git cl split.

R=peter@chromium.org

Change-Id: I027ae7ed7eb22e989a0e5a759177b6808762bcf7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2435229
Auto-Submit: Kevin McNee <mcnee@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813700}

components/gcm_driver/gcm_client_impl_unittest.cc

@@ -351,7 +351,7 @@ class GCMClientImplTest : public testing::Test,
     return gcm_client_->state_;
   }
   FakeMCSClient* mcs_client() const {
-    return reinterpret_cast<FakeMCSClient*>(gcm_client_->mcs_client_.get());
+    return static_cast<FakeMCSClient*>(gcm_client_->mcs_client_.get());
   }
   ConnectionFactory* connection_factory() const {
     return gcm_client_->connection_factory_.get();