Corrected SHA1 computation for AppRTC.

I noticed prebuilt_apprtc.zip kept getting re-downloaded, and this is
why.

BUG=569494
R=kjellander@chromium.org

Review URL: https://codereview.chromium.org/1579523002 .
diff --git a/download_apprtc_appengine_and_mercurial.py b/download_apprtc_appengine_and_mercurial.py
index 853923e..6617329 100755
--- a/download_apprtc_appengine_and_mercurial.py
+++ b/download_apprtc_appengine_and_mercurial.py
@@ -45,7 +45,7 @@
   old_mercurial_sha1 = utils.ComputeSHA1(mercurial_tar_path)
 
   apprtc_zip_path = os.path.join(webrtc_deps_path, 'prebuilt_apprtc.zip')
-  old_apprtc_sha1 = utils.ComputeSHA1(mercurial_tar_path)
+  old_apprtc_sha1 = utils.ComputeSHA1(apprtc_zip_path)
 
   _DownloadResources(webrtc_deps_path)