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/offline_pages
This CL was uploaded by git cl split.

R=jianli@chromium.org

Change-Id: Ifff21514df23ac39f13dff3228bd619760d8a679
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2434628
Auto-Submit: Kevin McNee <mcnee@chromium.org>
Reviewed-by: Jian Li <jianli@chromium.org>
Commit-Queue: Jian Li <jianli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811325}

components/offline_pages/core/background/request_coordinator_unittest.cc

@@ -187,7 +187,7 @@ class RequestCoordinatorTest : public testing::Test {
   }
   OfflinerStub* offliner() const { return offliner_; }
   SchedulerStub* scheduler_stub() const {
-    return reinterpret_cast<SchedulerStub*>(coordinator()->scheduler());
+    return static_cast<SchedulerStub*>(coordinator()->scheduler());
   }
   RequestQueue* queue() {
     return coordinator_taco_->request_coordinator()->queue_for_testing();