DevTools: Fix compile on Win with GN arg debug_devtools=true

Change-Id: Ie47791cbbf4bcbf23a7bbdb3622440f5693727dd
Reviewed-on: https://chromium-review.googlesource.com/c/1260342Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596453}

chrome/browser/ui/webui/devtools_ui.cc

@@ -8,6 +8,7 @@
 #include <utility>
 
 #include "base/command_line.h"
+#include "base/files/file_util.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted_memory.h"
 #include "base/path_service.h"