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}

webrtc/base/win32.h

@@ -86,7 +86,7 @@ bool Utf8ToWindowsFilename(const std::string& utf8, std::wstring* filename);
 
 // Convert a FILETIME to a UInt64
 inline uint64 ToUInt64(const FILETIME& ft) {
-  ULARGE_INTEGER r = {ft.dwLowDateTime, ft.dwHighDateTime};
+  ULARGE_INTEGER r = {{ft.dwLowDateTime, ft.dwHighDateTime}};
   return r.QuadPart;
 }
 

webrtc/modules/desktop_capture/desktop_frame_win.cc

@@ -35,7 +35,7 @@ DesktopFrameWin* DesktopFrameWin::Create(DesktopSize size,
   int bytes_per_row = size.width() * kBytesPerPixel;
 
   // Describe a device independent bitmap (DIB) that is the size of the desktop.
-  BITMAPINFO bmi = {0};
+  BITMAPINFO bmi = {};
   bmi.bmiHeader.biHeight = -size.height();
   bmi.bmiHeader.biWidth = size.width();
   bmi.bmiHeader.biPlanes = 1;

webrtc/system_wrappers/source/clock.cc

@@ -155,9 +155,9 @@ class WindowsRealTimeClock : public RealTimeClock {
   }
 
   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