FrameSelection::SelectWordAroundCaret(): Check Start and End for Null

EphemeralRange crashes when it receives a null position. If a
textfield is empty, the start and/or end of the word around the caret
may be null. This causes the renderer to crash.

Add a check that the start and end positions are not null before
passing them in to EphemeralRange.

Bug: 872443
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I7156bb99b53134d076359f9ea320b5a5cdbe5bc0
Reviewed-on: https://chromium-review.googlesource.com/1169584
Commit-Queue: Tessa Nijssen <tnijssen@google.com>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582264}

third_party/blink/renderer/core/editing/frame_selection.cc

@@ -1096,6 +1096,13 @@ bool FrameSelection::SelectWordAroundCaret() {
     // for avoiding unnecessary canonicalization.
     VisiblePosition start = StartOfWord(position, word_side);
     VisiblePosition end = EndOfWord(position, word_side);
+
+    // TODO(editing-dev): |StartOfWord()| and |EndOfWord()| should not make null
+    // for non-null parameter.
+    // See http://crbug.com/872443
+    if (start.DeepEquivalent().IsNull() || end.DeepEquivalent().IsNull())
+      continue;
+
     String text =
         PlainText(EphemeralRange(start.DeepEquivalent(), end.DeepEquivalent()));
     if (!text.IsEmpty() && !IsSeparator(text.CharacterStartingAt(0))) {