Use weak pointer to reference SpellCheckRequest from outside Blink

Blink Oilpan can't automatically remove pending spellcheck requests of a
closed document, as the requests are referenced by SpellCheckProvider,
which is out of Blink. This may become a leak source as revealed by
crbug.com/855383.

This patch prevents leak by changing the reference into a weak pointer.

Bug: 855383
Cq-Include-Trybots: luci.chromium.try:linux_chromium_asan_rel_ng
Change-Id: Ibdd535ddb51351a3e30776812dcd6ac4234feae3
Reviewed-on: https://chromium-review.googlesource.com/c/1428085
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625260}
3 files changed