Some API functions have no context and debug::ScopeIterator::
CreateForFunction is crashing on attempt to get context.

R=jgruber@chromium.org

Bug: chromium:759913
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I0a9861ea2d19bceff97c4394b34a8dda45222b78
Reviewed-on: https://chromium-review.googlesource.com/661789
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47981}