chromium68: bump revision

chromium68: bump revision

Commits included are backports from Chromium/Wayland upstream which
are part of the on-going work carried on SPEC-2266:

6a0470f2d03 [BACKPORT] [ozone/wayland] rely on implicit sync for Broadcom and Intel GPUs.
55acfb83ea4 [BACKPORT] [ozone/wayland] Use gbm with in-process-gpu mode
d58f1321884 [BACKPORT] ozone: xkbcommon: Pre-compute masks when setting keymap
ce84756108d [BACKPORT] ozone/wayland: xkbcommon: Fix layout switching
5e8508b280e [BACKPORT] ozone/wayland: Support NumLock in non-chromeos builds
7e90082ccfc [BACKPORT] ozone/wayland: Remove WaylandXkbKeyboardLayoutEngine
c74c5365d40 [BACKPORT] ozone/wayland: Fix NEO keyboards layout handling
1055757769d [BACKPORT] ozone/wayland: Support dead keys
e2648e9c17b [BACKPORT] Move CharacterComposer into //ui/base/ime

Besides:

* 1055757769d [BACKPORT] ozone/wayland: Support dead keys
  fixes SPEC-2746
* 55acfb83ea4 [BACKPORT] [ozone/wayland] Use gbm with in-process-gpu mode
  fixes SPEC-2699

Bug-AGL: SPEC-2266
Bug-AGL: SPEC-2746
Bug-AGL: SPEC-2699

Change-Id: I19eaa6bf225ac478386c53b94955ccb0a4856e62
Signed-off-by: Antia Puentes <apuentes@igalia.com>