DevTools: prepare for deleting scriptPreprocessor parameter

ScriptPreprocessor is being deprecated and will be removed soon. This CL prepares page_handler for removing scriptPreprocessor parameter from Page.reload command.

BUG=394230, 438626

Review URL: https://codereview.chromium.org/898433002

Cr-Commit-Position: refs/heads/master@{#314290}

content/browser/devtools/protocol/page_handler.h

@@ -45,7 +45,7 @@ class PageHandler {
 
   Response Reload(const bool* ignoreCache,
                   const std::string* script_to_evaluate_on_load,
-                  const std::string* script_preprocessor);
+                  const std::string* script_preprocessor = NULL);
 
   Response Navigate(const std::string& url, FrameId* frame_id);