chromeos: disable CompositingRenderWidgetHostViewBrowserTestHiDPI for mus

This makes it possible to turn on content_browsertests with --mus

BUG=785536
TEST=test only change
TBR=sadrul@chromium.org

Change-Id: I92ac83e6ff596f270947a75956f760bef79017d8
Reviewed-on: https://chromium-review.googlesource.com/849656Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526889}

content/browser/renderer_host/render_widget_host_view_browsertest.cc

@@ -47,6 +47,10 @@
 #include "ui/gfx/geometry/size_conversions.h"
 #include "ui/gl/gl_switches.h"
 
+#if defined(USE_AURA)
+#include "ui/aura/env.h"
+#endif
+
 #if defined(OS_WIN)
 #include "base/win/windows_version.h"
 #endif
@@ -1031,6 +1035,11 @@ class CompositingRenderWidgetHostViewBrowserTestHiDPI
 
 IN_PROC_BROWSER_TEST_P(CompositingRenderWidgetHostViewBrowserTestHiDPI,
                        ScrollOffset) {
+// TODO(sadrul): enable for mus: http://crbug.com/798904.
+#if defined(USE_AURA)
+  if (aura::Env::GetInstance()->mode() == aura::Env::Mode::MUS)
+    return;
+#endif
   const int kContentHeight = 2000;
   const int kScrollAmount = 100;