If context is not available during nearHeapLimitCallback then request
break inside any context group.

R=dgozman@chromium.org

Bug: chromium:846311
Cq-Include-Trybots: luci.chromium.try:linux_chromium_headless_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I2b7dcda8e9758672f27c15ce18620bd57c4152c0
Reviewed-on: https://chromium-review.googlesource.com/1129100Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54337}