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This CL adds to RenderMenuList::setTextFromOption(int) so that when the
associated HTML <select> element has the multiple attribute, the <select>
element's options are iterated over and a count of the number of selected
options is determined. When the number of selected options is 0 or 2 or
more, then the RenderMenuList's text is set to "SELECTED_COUNT selected"
(e.g. "2 selected"). When exactly one option is selected, then the text
is set to the selected option's text.

This is similar to iOS. When 0 or 2 or more options are selected, then
the text of the menulist button is set to "SELECTED_COUNT Items". When
exactly one option is selected, then the text is set to the selected
option's text.

Depends on:
- https://codereview.chromium.org/280483002/
- https://codereview.chromium.org/295933003/

BUG=314704

Review URL: https://codereview.chromium.org/302473003

git-svn-id: svn://svn.chromium.org/blink/trunk@175010 bbb929c8-8fbe-4397-9dbb-9b2b20218538