Fix one more -Wshorten-64-to-32 warning in chrome_elf

../../chrome_elf/chrome_elf_test_stubs.cc(91,18):  warning: implicit conversion loses integer precision: 'long long' to 'uint32_t' (aka 'unsigned int') [-Wshorten-64-to-32]
  return tracker - buffer;
  ~~~~~~ ~~~~~~~~^~~~~~~~

BUG=879657

Change-Id: Ie9044aae8fffebd05fa45c26ee5a63649ea228f5
Reviewed-on: https://chromium-review.googlesource.com/1214743Reviewed-by: Robert Shield <robertshield@chromium.org>
Commit-Queue: Robert Shield <robertshield@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589767}

chrome_elf/chrome_elf_test_stubs.cc

@@ -4,6 +4,7 @@
 
 #include "base/command_line.h"
 #include "base/files/file_path.h"
+#include "base/numerics/safe_conversions.h"
 #include "base/path_service.h"
 #include "base/stl_util.h"
 #include "base/win/windows_types.h"
@@ -88,7 +89,8 @@ uint32_t DrainLog(uint8_t* buffer,
 
     tracker += entry_size;
   }
-  return tracker - buffer;
+
+  return base::checked_cast<uint32_t>(tracker - buffer);
 }
 
 bool RegisterLogNotification(HANDLE event_handle) {