All builders that used level 1 should meanwhile define
v8_enable_slow_dchecks, and so they can use level 2 without losing
coverage. Level 2, however, is considerably faster, so we want to use it
on those builders as well. The make optdebug setting is not affected by
this change.

BUG=none
R=machenbach@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/847753002

Cr-Commit-Position: refs/heads/master@{#26019}