Previously, FrameFunctionIterator::next() assumed that the frame summary
was non-empty. It's now possible for the list not to be empty, if the
JS microtask pump invokes a builtin function which uses
FrameFunctionIterator directly. While this is unlikely to show up in
real world code, it is necessary to handle it to prevent crashes.

BUG=chromium:794744
R=mstarzinger@chromium.org, cbruni@chromium.org, verwaest@chromium.org

Change-Id: Ie95c2228544f57730d1c6c1ff955b2c94ff1c06b
Reviewed-on: https://chromium-review.googlesource.com/833266Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#50221}