DevTools: fix null-pointer exception with sourcemap infobar

Ther sourcemap infobar might never be created if user has clicked
"never show" button.

R=dgozman, einbinder

Change-Id: I5d494ca218bb3b793a0c734d5fcf92549821749d
Reviewed-on: https://chromium-review.googlesource.com/1018530Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552072}

third_party/blink/renderer/devtools/front_end/sources/DebuggerPlugin.js

@@ -1412,6 +1412,8 @@ Sources.DebuggerPlugin = class extends Sources.UISourceCodeFrame.Plugin {
     this._sourceMapInfobar = UI.Infobar.create(
         UI.Infobar.Type.Info, Common.UIString('Source Map detected.'),
         Common.settings.createSetting('sourceMapInfobarDisabled', false));
+    if (!this._sourceMapInfobar)
+      return;
     this._sourceMapInfobar.createDetailsRowMessage(Common.UIString(
         'Associated files should be added to the file tree. You can debug these resolved source files as regular JavaScript files.'));
     this._sourceMapInfobar.createDetailsRowMessage(Common.UIString(