Correct usage of text iterator in idle time spellchecker

Idle time spell checker used to assume that TextIterator::advance()
is no-op when the iterator is already at end, which is incorrect.

This patch fixes the issue by adding |atEnd()| checks.

Note: When at end, TextIterator::advance() should be no-op but is not.
See crbug.com/699747

BUG=679616
TEST=compositing/overflow/do-not-paint-outline-into-composited-scrolling-contents.html

Review-Url: https://codereview.chromium.org/2734393003
Cr-Commit-Position: refs/heads/master@{#455654}

third_party/WebKit/Source/core/editing/spellcheck/HotModeSpellCheckRequester.cpp

@@ -73,11 +73,13 @@ EphemeralRange calculateHotModeCheckingRange(const Element& editable,
                                       .build();
   BackwardsCharacterIterator backwardIterator(fullRange.startPosition(),
                                               position, behavior);
-  backwardIterator.advance(kHotModeChunkSize / 2);
+  if (!backwardIterator.atEnd())
+    backwardIterator.advance(kHotModeChunkSize / 2);
   const Position& chunkStart = backwardIterator.endPosition();
   CharacterIterator forwardIterator(position, fullRange.endPosition(),
                                     behavior);
-  forwardIterator.advance(kHotModeChunkSize / 2);
+  if (!forwardIterator.atEnd())
+    forwardIterator.advance(kHotModeChunkSize / 2);
   const Position& chunkEnd = forwardIterator.endPosition();
   return expandRangeToSentenceBoundary(EphemeralRange(chunkStart, chunkEnd));
 }