Fix rtl text shifting in PopupListBox

What is happening is:
1. User opens popup
2. PopupListBox::layout measure's row width
  2-1. text width is calculated as ltr
  2-2. this width for ltr is registered to the WidthCache
3. PopupListBox::paintRow is called
  3-1. WidthCache has a interval mechanism so the cached ltr width is not used
  3-2. text width is calculated as rtl
  3-3. Text is  drawn as rtl
4. User hovers over items
5. PopupListBox::paintRow is called each time the selection changes
  5-1. WidthCache's cached ltr width is used
  5-2. Text is  drawn as rtl (rtl width is shorter than the rtl width so there is extra space on the right side)

There are two problems
- PopupListBox::layout is measuring row width using ltr always
- WidthCache is returning ltr width for rtl text

This just fixes PopupListBox::layout.
BUG=398929

Review URL: https://codereview.chromium.org/451983004

git-svn-id: svn://svn.chromium.org/blink/trunk@179906 bbb929c8-8fbe-4397-9dbb-9b2b20218538