Add observers for DIP scale change.

content/browser/android/content_view_core_impl.cc

Index: content/browser/android/content_view_core_impl.cc
diff --git a/content/browser/android/content_view_core_impl.cc b/content/browser/android/content_view_core_impl.cc
index e627fcd7611712cccc0a78f09b97a433a233f072..1efa87f16808aaa8aea2e2a832030c9fe9457bcd 100644
--- a/content/browser/android/content_view_core_impl.cc
+++ b/content/browser/android/content_view_core_impl.cc
@@ -884,6 +884,12 @@ void ContentViewCoreImpl::SetFocus(JNIEnv* env,
   SetFocusInternal(focused);
 }
 
+void ContentViewCoreImpl::SetDIPScale(JNIEnv* env,
+                                      const JavaParamRef<jobject>& obj,
+                                      jfloat dipScale) {
+  dpi_scale_ = dipScale;

[boliu, 2016/09/14 04:35:32]

I remember making this comment before. What's responsible for updating all the
places where this is used?

[Tima Vaisburd, 2016/09/15 00:05:03]

I probably never understood completely, so let me ask a more detailed question:
are you talking about the member variables that store the result which includes
DIP scale and thus depend on it indirectly?

For instance, OnSelectionEvent() scales the selection rect up with DIP scale and
passes it to Java side where it is stored in mSelectionRect. mSelectionRect
should be recalculated unless the selection goes away during DIP scale changing
process.

This job is not done.

Better yet, as much as possible should be in DIP pixels.

[boliu, 2016/09/15 05:27:41]

Yep
Yeah, moving all that conversion to java would be one super big help. I guess
you can either leave that as a TODO in this CL, or maybe do that refactor first.

+}
+
 void ContentViewCoreImpl::SetFocusInternal(bool focused) {
   if (!GetRenderWidgetHostViewAndroid())
     return;