Tentatively re-add UTF string conversion header to fix compile err

BUG=1143546,1145278
R=sky@chromium.org,bbudge@chromium.org

Change-Id: I23be33444805035442f858e84a72d0f05465e964
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518268Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823695}

chrome/browser/chrome_plugin_browsertest.cc

@@ -10,6 +10,7 @@
 #include "base/callback.h"
 #include "base/command_line.h"
 #include "base/process/process.h"
+#include "base/strings/utf_string_conversions.h"
 #include "build/branding_buildflags.h"
 #include "build/build_config.h"
 #include "chrome/test/base/in_process_browser_test.h"