This adds an entry to chrome://flags that allows layer borders to be
enabled for Chrome OS UI.

It also makes the debug border setting a bitset so borders for individual
layer types can be enabled. This makes the debug flag more useful when
debugging a specific layer type (e.g. render surface layers).

BUG=678236
TEST=chrome --ui-show-composited-layer-borders=renderpass
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2795703002
Cr-Commit-Position: refs/heads/master@{#464640}