Fix the output path for webui_test_resources.h.

Review-Url: https://codereview.chromium.org/2287223003
Cr-Commit-Position: refs/heads/master@{#415173}

chrome/test/data/BUILD.gn

@@ -7,7 +7,7 @@ import("//tools/grit/grit_rule.gni")
 grit("webui_test_resources") {
   source = "webui_test_resources.grd"
   outputs = [
-    "chrome/test/data/grit/webui_test_resources.h",
+    "test/data/grit/webui_test_resources.h",
     "webui_test_resources.pak",
   ]
   output_dir = "$root_gen_dir/chrome"

chrome/test/data/webui_test_resources.grd

 <?xml version="1.0" encoding="UTF-8"?>
 <grit latest_public_release="0" current_release="1" output_all_resource_defines="false">
   <outputs>
-    <output filename="chrome/test/data/grit/webui_test_resources.h" type="rc_header">
+    <output filename="test/data/grit/webui_test_resources.h" type="rc_header">
       <emit emit_type='prepend'></emit>
     </output>
     <output filename="webui_test_resources.pak" type="data_package" />