Removed superfluous custom style DCHECK.

Custom style callbacks are called during style recalc. There is no need
to check that ShadowRoot does not have such callbacks during layout
tree rebuild. There is already a check in ShadowRoot::RecalcStyle().

R=nainar@chromium.org
BUG=595137

Review-Url: https://codereview.chromium.org/2833223002
Cr-Commit-Position: refs/heads/master@{#466551}
1 file changed