Add composition{Start,End}Offset attributes to InputMethodContext.

Add composition{Start,End}Offset attributes to InputMethodContext,
as written in the latest Editor's draft of IME API:
https://dvcs.w3.org/hg/ime-api/raw-file/71a77f3766d2/Overview.html

Currently it works properly for <input> and <textarea>.

When this is used for contenteditable, it may return
bogus value, and it is being discussed at
https://www.w3.org/Bugs/Public/show_bug.cgi?id=22093

BUG=238585
TEST=LayoutTest fast/ime/composition_offsets.html

Review URL: https://codereview.chromium.org/102853004

git-svn-id: svn://svn.chromium.org/blink/trunk@163853 bbb929c8-8fbe-4397-9dbb-9b2b20218538