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.

/content/browser/media
This CL was uploaded by git cl split.

R=sandersd@chromium.org

Change-Id: I324ae893245bbd253e68e9e2d7b31fe0c62ccb87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2434909
Auto-Submit: Kevin McNee <mcnee@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811350}

content/browser/media/audio_stream_monitor_unittest.cc

@@ -52,7 +52,7 @@ class AudioStreamMonitorTest : public RenderViewHostTestHarness {
   void SetUp() override {
     RenderViewHostTestHarness::SetUp();
 
-    WebContentsImpl* web_contents = reinterpret_cast<WebContentsImpl*>(
+    WebContentsImpl* web_contents = static_cast<WebContentsImpl*>(
         RenderViewHostTestHarness::web_contents());
     web_contents->SetDelegate(&mock_web_contents_delegate_);