Apply Position::IsValidFor() at EphemeralRange constructor.

We check more exactly.
Especially this is important for PositionInFlatTree.


Bug: 
Change-Id: I43edef60ca3b966cb5501eef241f12d99f40e0ae
Reviewed-on: https://chromium-review.googlesource.com/662718Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501217}

third_party/WebKit/Source/core/editing/EphemeralRange.cpp

@@ -38,9 +38,9 @@ EphemeralRangeTemplate<Strategy>::EphemeralRangeTemplate(
     return;
   }
   DCHECK(end_position_.IsNotNull());
+  DCHECK(start_position_.IsValidFor(*start_position_.GetDocument()));
+  DCHECK(end_position_.IsValidFor(*end_position_.GetDocument()));
   DCHECK_EQ(start_position_.GetDocument(), end_position_.GetDocument());
-  DCHECK(start_position_.IsConnected());
-  DCHECK(end_position_.IsConnected());
   DCHECK_LE(start_position_, end_position_);
 }