Initialize |selection_popup_controller_| properly

Set the class var to nullptr in the constructor. Missing initialization
is suspected to cause crash.

BUG=715826

Review-Url: https://codereview.chromium.org/2842373002
Cr-Commit-Position: refs/heads/master@{#467618}

content/browser/renderer_host/render_widget_host_view_android.cc

@@ -446,6 +446,7 @@ RenderWidgetHostViewAndroid::RenderWidgetHostViewAndroid(
       is_in_vr_(false),
       content_view_core_(nullptr),
       ime_adapter_android_(nullptr),
+      selection_popup_controller_(nullptr),
       background_color_(SK_ColorWHITE),
       cached_background_color_(SK_ColorWHITE),
       view_(this),