[Devtools] Replace "Stacks" with "Causes"

This patch switches to "Causes" for explaining timeline records. The
major change is "Stacks" have been renamed "Causes". This is prepwork
for two upcoming projects: invalidation tracking which will fill out many
more causes, and javascript samples which are easily confused with
the "stack" term. If the causes checkbox is not checked, no initiator
or stack information will be shown.

Some minor rewording of the causes has also been done. For example,
the stack for a layout now reads "First layout invalidation" instead of
"Layout invalidated".

A screenshot is available at http://pr.gg/justcause3.png

BUG=410701

Review URL: https://codereview.chromium.org/631573002

git-svn-id: svn://svn.chromium.org/blink/trunk@183626 bbb929c8-8fbe-4397-9dbb-9b2b20218538