Fix some clang warnings with -Wmissing-braces in v8.

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.

BUG=chromium:505297
LOG=n

Review URL: https://codereview.chromium.org/1219233003

Cr-Commit-Position: refs/heads/master@{#29437}

src/base/sys-info.cc

@@ -44,7 +44,7 @@ int SysInfo::NumberOfProcessors() {
   }
   return static_cast<int>(result);
 #elif V8_OS_WIN
-  SYSTEM_INFO system_info = {0};
+  SYSTEM_INFO system_info = {};
   ::GetNativeSystemInfo(&system_info);
   return static_cast<int>(system_info.dwNumberOfProcessors);
 #endif