Enable size_t to int truncation warnings in PPAPI

ppapi/shared_impl/flash_clipboard_format_registry.cc

Index: ppapi/shared_impl/flash_clipboard_format_registry.cc
diff --git a/ppapi/shared_impl/flash_clipboard_format_registry.cc b/ppapi/shared_impl/flash_clipboard_format_registry.cc
index 89a051e2d3d9c5d5c6f833b38c9efedec531d569..fa43478a33954364579ce75d626768ef25c1aae1 100644
--- a/ppapi/shared_impl/flash_clipboard_format_registry.cc
+++ b/ppapi/shared_impl/flash_clipboard_format_registry.cc
@@ -6,6 +6,8 @@
 
 #include <cctype>
 
+#include "base/numerics/safe_conversions.h"
+
 namespace ppapi {
 
 namespace {
@@ -43,7 +45,8 @@ uint32_t FlashClipboardFormatRegistry::RegisterFormat(
       custom_formats_.size() > kMaxNumFormats) {
     return PP_FLASH_CLIPBOARD_FORMAT_INVALID;
   }
-  uint32_t key = kFirstCustomFormat + custom_formats_.size();
+  uint32_t key = kFirstCustomFormat +
+                 base::checked_cast<uint32_t>(custom_formats_.size());
   custom_formats_[key] = format_name;
   return key;
 }