Fix two simple type mismatches thay may cause compilation issues on win.

BUG=None

Review-Url: https://codereview.webrtc.org/2955193002
Cr-Commit-Position: refs/heads/master@{#18836}

webrtc/modules/video_coding/codecs/tools/video_quality_measurement.cc

@@ -176,7 +176,7 @@ int HandleCommandLineFlags(webrtc::test::TestConfig* config) {
   if (FLAGS_output_filename.empty()) {
     // Cut out the filename without extension from the given input file
     // (which may include a path)
-    int startIndex = FLAGS_input_filename.find_last_of("/") + 1;
+    size_t startIndex = FLAGS_input_filename.find_last_of("/") + 1;
     if (startIndex == 0) {
       startIndex = 0;
     }