Rename cc::ScrollNode's inner and outer viewport members

ScrollNode has members for is_{inner, outer}_viewport_scroll_layer but
these names do not make sense without ScrollNode's owning_layer_id
concept. This patch renames these members to scrolls_inner_viewport
and scrolls_outer_viewport.

In addition, this patch cleans up viewport layer detection added in
https://codereview.chromium.org/2707783003 to use these new members.

BUG=693740
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2714713003
Cr-Commit-Position: refs/heads/master@{#452897}
6 files changed