[DevTools] use debug_frontend flag in content_shell for tests

Without this flag InspectorFrontendHost can call callbacks while devtools on pause. It makes debugging of layout tests useless.

R=dgozman@chromium.org, pfeldman@chromium.org

Review URL: https://codereview.chromium.org/1639973003

Cr-Commit-Position: refs/heads/master@{#371919}

content/content_shell.gypi

@@ -281,6 +281,11 @@
             '../components/components.gyp:breakpad_host',
           ],
         }],
+        ['debug_devtools==1', {
+          'defines': [
+            'DEBUG_DEVTOOLS=1',
+          ],
+        }],
         ['(OS=="linux" or OS=="android") and use_allocator!="none"', {
           'dependencies': [
             # This is needed by content/app/content_main_runner.cc

content/shell/BUILD.gn

@@ -176,6 +176,8 @@ static_library("content_shell_lib") {
 
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     "//build/config/compiler:no_size_t_to_int_warning",
+
+    "//third_party/WebKit/public:debug_devtools",
   ]
 
   defines = [ "CONTENT_SHELL_VERSION=\"$content_shell_version\"" ]

content/shell/browser/layout_test/layout_test_devtools_frontend.cc

@@ -53,11 +53,14 @@ GURL LayoutTestDevToolsFrontend::GetDevToolsPathAsURL(
       dir_exe.AppendASCII("resources/inspector/inspector.html");
 
   GURL result = net::FilePathToFileURL(dev_tools_path);
+  std::string url_string =
+      base::StringPrintf("%s?experiments=true", result.spec().c_str());
+#if defined(DEBUG_DEVTOOLS)
+  url_string += "&debugFrontend=true";
+#endif  // defined(DEBUG_DEVTOOLS)
   if (!settings.empty())
-    result = GURL(base::StringPrintf("%s?settings=%s&experiments=true",
-                                     result.spec().c_str(),
-                                     settings.c_str()));
-  return result;
+    url_string += "&settings=" + settings;
+  return GURL(url_string);
 }
 
 void LayoutTestDevToolsFrontend::ReuseFrontend(const std::string& settings,