clang/win: Let some chromium_code targets build with -Wextra.

content/child/npapi/webplugin_ime_win.cc

Index: content/child/npapi/webplugin_ime_win.cc
diff --git a/content/child/npapi/webplugin_ime_win.cc b/content/child/npapi/webplugin_ime_win.cc
index 7ff6a5d75156cd2706738c6b1b6eb80d3d083b26..e095ee1f780653bc820ab6a284e5fcbf28a7a07a 100644
--- a/content/child/npapi/webplugin_ime_win.cc
+++ b/content/child/npapi/webplugin_ime_win.cc
@@ -169,7 +169,7 @@ FARPROC WebPluginIMEWin::GetProcAddress(LPCSTR name) {
         reinterpret_cast<FARPROC>(ImmSetCandidateWindow) },
     { "ImmSetOpenStatus", reinterpret_cast<FARPROC>(ImmSetOpenStatus) },
   };
-  for (int i = 0; i < arraysize(kImm32Functions); ++i) {
+  for (size_t i = 0; i < arraysize(kImm32Functions); ++i) {
     if (!lstrcmpiA(name, kImm32Functions[i].name))
       return kImm32Functions[i].function;
   }