Fix some clang warnings with -Wmissing-braces in WebRTC.

Clang warns if there are missing braces around a subobject
initializer. The most common idiom that triggers this is:
  STRUCT s = {0};
if the first field of STRUCT is itself a struct. This can
be more simply written as:
  STRUCT s = {};
which also prevents the warning from firing.

Other instances of the warning have been fixed by adding
braces where appropriate.

BUG=505297
TBR=jiayl@webrtc.org

Review URL: https://codereview.webrtc.org/1216353002

Cr-Commit-Position: refs/heads/master@{#9529}
diff --git a/webrtc/system_wrappers/source/clock.cc b/webrtc/system_wrappers/source/clock.cc
index a7f261a..31c5226 100644
--- a/webrtc/system_wrappers/source/clock.cc
+++ b/webrtc/system_wrappers/source/clock.cc
@@ -155,9 +155,9 @@
   }
 
   static ReferencePoint GetSystemReferencePoint() {
-    ReferencePoint ref = {0};
-    FILETIME ft0 = {0};
-    FILETIME ft1 = {0};
+    ReferencePoint ref = {};
+    FILETIME ft0 = {};
+    FILETIME ft1 = {};
     // Spin waiting for a change in system time. As soon as this change happens,
     // get the matching call for timeGetTime() as soon as possible. This is
     // assumed to be the most accurate offset that we can get between