Add CompositionMarker (subclass of DocumentMarker)

This CL moves the Composition marker-specific functionality of DocumentMarker
into a new CompositionMarker subclass (similar to TextMatchMarker). I will do
the same for Spelling and Grammar markers in future CLs.

BUG=707867
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2908643002
Cr-Commit-Position: refs/heads/master@{#476220}
9 files changed