Attempt to stabilize inspector-protocol/layers/get-layers.html

... by removing output while running the essential parts of the
test, as it may introduce additional layers at unpredictable moments.

BUG=313922
TBR=shawnsingh,pfeldman,yurys

Review URL: https://codereview.chromium.org/54533004

git-svn-id: svn://svn.chromium.org/blink/trunk@161158 bbb929c8-8fbe-4397-9dbb-9b2b20218538