[LayoutNG] Fix 1px failure in WebFrameTest

Unit tests WebFrameTest.MoveRangeSelectionExtent and MoveRangeSelectionExtentCannotCollapse currently fail with LayoutNG on Mac due to a 1px rounding difference when calculating caret rects (InlineTextBox::PositionForOffset() vs. NGPhysicalTextFragment::InlinePositionForOffset()).

As the legacy caret rect rounding process is already very artificial, it's not worthy recovering exactly the same process in NG. Hence, this patch simply tweaks hit test coordinates in the unit tests to make them pass with LayoutNG.

Bug: 591099
Change-Id: Ia0eb2ecdde0eb593ac72e010822e780720b930da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1612118
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659853}

third_party/blink/renderer/core/exported/web_frame_test.cc

@@ -5426,7 +5426,9 @@ static WebPoint BottomRightMinusOne(const WebRect& rect) {
   // selection bounds, selectRange() will select the *next* element. That's
   // strictly correct, as hit-testing checks the pixel to the lower-right of
   // the input coordinate, but it's a wart on the API.
-  return WebPoint(rect.x + rect.width - 1, rect.y + rect.height - 1);
+  if (rect.width > 0)
+    return WebPoint(rect.x + rect.width - 1, rect.y + rect.height - 1);
+  return WebPoint(rect.x + rect.width, rect.y + rect.height - 1);
 }
 
 static WebRect ElementBounds(WebLocalFrame* frame, const WebString& id) {