Add more logging in DoLaunchChildTestProcess().

When it calls ReadFileToString() and fails, log the name of the file it
is trying to read.

BUG=826408

Change-Id: I546977b985c71cbd631820e7d7950b7f7449faba
Reviewed-on: https://chromium-review.googlesource.com/1024743Reviewed-by: Julia Tuttle <juliatuttle@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553188}

base/test/launcher/test_launcher.cc

@@ -520,7 +520,8 @@ void DoLaunchChildTestProcess(
   if (redirect_stdio) {
     fflush(output_file.get());
     output_file.reset();
-    CHECK(ReadFileToString(output_filename, &output_file_contents));
+    CHECK(ReadFileToString(output_filename, &output_file_contents))
+        << output_filename;
 
     if (!DeleteFile(output_filename, false)) {
       // This needs to be non-fatal at least for Windows.