DevTools: move front-end URL handling to DevToolsUIBindingds

BUG=662859

Review-Url: https://codereview.chromium.org/2607833002
Cr-Commit-Position: refs/heads/master@{#440926}
void DevToolsUIBindings::DidNavigateMainFrame() {
  frontend_loaded_ = false;
}

void DevToolsUIBindings::DidNavigateMainFrame() {
  frontend_loaded_ = false;
}
