Oilpan: fix build after r175261.

Add required parens.

R=
BUG=378167
NOTRY=true

Review URL: https://codereview.chromium.org/311563006

git-svn-id: svn://svn.chromium.org/blink/trunk@175268 bbb929c8-8fbe-4397-9dbb-9b2b20218538