Rename selection{Start,End} properties to activeSegment{Start,End}.
Merge Composition interface to CompositionEvent.

Renaming 'selection{Start,End}' to 'activeSegment{Start,End}'
was discussed at:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=22059

Merging Composition interface into CompositionEvent is discussed at:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=23830
and the resulting spec is:
https://dvcs.w3.org/hg/ime-api/raw-file/default/Annex.html

BUG=238585

Review URL: https://codereview.chromium.org/117013004

git-svn-id: svn://svn.chromium.org/blink/trunk@164575 bbb929c8-8fbe-4397-9dbb-9b2b20218538