Remove aura only files from non-aura content/renderer

Some mostly harmless *_aura* files were included by mistake
in the non-aura build of content/renderer. They turned out to not be
quite so harmless in a jumbo build if combined with the non-aura
version of the same code.

Change-Id: Idfe65638ca16a2e700b485acd67f253073ae5867
Reviewed-on: https://chromium-review.googlesource.com/1102426Reviewed-by: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#567961}

content/renderer/BUILD.gn

@@ -806,7 +806,7 @@ target(link_target_type, "renderer") {
     ]
   }
 
-  if (is_mac) {
+  if (is_mac || !use_aura) {
     sources -= [
       "webscrollbarbehavior_impl_aura.cc",
       "webscrollbarbehavior_impl_aura.h",