Reason for this is that RenderTextHarfBuzz::FindCursorPosition does not always return a valid grapheme boundary index. As a result, RenderTextHarfBuzz::MoveCursor fails.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4e4ca175171eb6c46603799904a624e6797773e4

commit 4e4ca175171eb6c46603799904a624e6797773e4
Author: karandeepb <karandeepb@chromium.org>
Date: Wed Jan 04 02:12:24 2017

RenderTextHarfBuzz: Add support for multi line text selection.

This CL-
  - Adds support for multi-line text selection to RenderTextHarfBuzz. To do this
    RenderTextHarfBuzz::FindCursorPosition and
    RenderTextHarfBuzz::GetSubstringBounds are reimplemented.
  - RenderTextHarfBuzz::FindCursorPosition now returns valid grapheme boundaries
    hence fixing  issue 673986 .
  - RenderTextHarfBuzz::GetSubstringBounds did already support multiline but the
    implementation was flawed. It relied on the text space bounds intersection
    computed from LineSegment's x_range paramater and from TextRunHarfBuzz.
    However these were not in sync if some newline segments were popped or if
    some text was truncated.
  - Enables multi-line text selection for views::Labels.
  - Corrects the behavior for RTL when mouse is dragged above/below the text
    bounds on MacViews.
  - Adds lots of tests.

BUG= 650120 ,  630365 ,  600166 ,  673986 
TEST= Open views_examples_with_content_exe. Select Labels from the dropdown.
Enter a large string in the "Content" textfield. Enable the Multiline and Text
Selection checkboxes. Ensure text selection works correctly on the Label.

Review-Url: https://codereview.chromium.org/2541313002
Cr-Commit-Position: refs/heads/master@{#441290}

[modify] https://crrev.com/4e4ca175171eb6c46603799904a624e6797773e4/ui/gfx/render_text.cc
[modify] https://crrev.com/4e4ca175171eb6c46603799904a624e6797773e4/ui/gfx/render_text.h
[modify] https://crrev.com/4e4ca175171eb6c46603799904a624e6797773e4/ui/gfx/render_text_harfbuzz.cc
[modify] https://crrev.com/4e4ca175171eb6c46603799904a624e6797773e4/ui/gfx/render_text_harfbuzz.h
[modify] https://crrev.com/4e4ca175171eb6c46603799904a624e6797773e4/ui/gfx/render_text_unittest.cc
[modify] https://crrev.com/4e4ca175171eb6c46603799904a624e6797773e4/ui/views/controls/label.cc
[modify] https://crrev.com/4e4ca175171eb6c46603799904a624e6797773e4/ui/views/controls/label.h
[modify] https://crrev.com/4e4ca175171eb6c46603799904a624e6797773e4/ui/views/controls/label_unittest.cc
[modify] https://crrev.com/4e4ca175171eb6c46603799904a624e6797773e4/ui/views/controls/textfield/textfield_unittest.cc
[modify] https://crrev.com/4e4ca175171eb6c46603799904a624e6797773e4/ui/views/examples/label_example.cc
[modify] https://crrev.com/4e4ca175171eb6c46603799904a624e6797773e4/ui/views/examples/label_example.h
[modify] https://crrev.com/4e4ca175171eb6c46603799904a624e6797773e4/ui/views/selection_controller.cc
[modify] https://crrev.com/4e4ca175171eb6c46603799904a624e6797773e4/ui/views/style/platform_style.cc
[modify] https://crrev.com/4e4ca175171eb6c46603799904a624e6797773e4/ui/views/style/platform_style.h
[modify] https://crrev.com/4e4ca175171eb6c46603799904a624e6797773e4/ui/views/style/platform_style_mac.mm