Move PageVisibilityState from child/ to main_thread/.

This patch moves the files for PageVisibilityState from scheduler/child/
to scheduler/main_thread/, as this enum is only used in the main thread
scheduler. There were stale #includes in workers implementation, but
they are no longer used, so they are removed.

At the same time, the content of default_params.h is merged into
page_visibility_state.h, since the header only contains one constant
of type PageVisiblityState, and it seems not used for other purposes.

Bug: 826203
Change-Id: Iea1a141802c4dbffba22000daaaf1bbce1b214ac
Reviewed-on: https://chromium-review.googlesource.com/1084575
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564418}
11 files changed