Remove empty-string comparisons.

Use .empty() and !.empty() in favor of == "" or != "".

BUG=
R=tommi@webrtc.org

Review URL: https://codereview.webrtc.org/1228913003

Cr-Commit-Position: refs/heads/master@{#9559}

webrtc/tools/agc/agc_test.cc

@@ -91,7 +91,7 @@ void RunAgc() {
   ASSERT_TRUE(out_file != NULL);
 
   int gain_map[256];
-  if (FLAGS_gain_file != "") {
+  if (!FLAGS_gain_file.empty()) {
     FILE* gain_file = fopen(FLAGS_gain_file.c_str(), "rt");
     ASSERT_TRUE(gain_file != NULL);
     ReadGainMapFromFile(gain_file, FLAGS_gain_offset, gain_map);