[sksg] Fix inval tracking in Group::removeChild()

TBR=
Change-Id: If3536f8be9d379402cde18c0482108fcf5ae01bb
Reviewed-on: https://skia-review.googlesource.com/158822
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/modules/sksg/src/SkSGGroup.cpp b/modules/sksg/src/SkSGGroup.cpp
index 4841714..df696c4 100644
--- a/modules/sksg/src/SkSGGroup.cpp
+++ b/modules/sksg/src/SkSGGroup.cpp
@@ -43,6 +43,7 @@
     fChildren.erase(std::remove(fChildren.begin(), fChildren.end(), node), fChildren.end());
     SkASSERT(fChildren.size() == origSize - 1);
 
+    this->unobserveInval(node);
     this->invalidate();
 }