Fixing remaining VC++ 2015 64-bit build breaks

chrome/app/chrome_watcher_command_line_win.cc

Index: chrome/app/chrome_watcher_command_line_win.cc
diff --git a/chrome/app/chrome_watcher_command_line_win.cc b/chrome/app/chrome_watcher_command_line_win.cc
index 12b3e143629eb6beedcafb0913f5d818e67849dc..af7dc95fc8e98ba85a51f8ebf9725b3e82ea1eaa 100644
--- a/chrome/app/chrome_watcher_command_line_win.cc
+++ b/chrome/app/chrome_watcher_command_line_win.cc
@@ -23,8 +23,11 @@ const char kMainThreadIdSwitch[] = "main-thread-id";
 void AppendHandleSwitch(const std::string& switch_name,
                         HANDLE handle,
                         base::CommandLine* command_line) {
-  command_line->AppendSwitchASCII(
-      switch_name, base::UintToString(reinterpret_cast<uint32_t>(handle)));
+  // Cast through uintptr_t and then uint32_t to make the truncation
+  // explicit. Handles are size-of-pointer but are always 32-bit values.
+  command_line->AppendSwitchASCII(switch_name,
+                                  base::UintToString(static_cast<uint32_t>(
+                                      reinterpret_cast<uintptr_t>(handle))));
 }
 
 uint32_t ReadUintSwitch(const base::CommandLine& command_line,