reflow comments in core/layout/compositing,core/observer

BUG=563793
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2393673004
Cr-Commit-Position: refs/heads/master@{#423297}
16 files changed