Adds check for service callback before enabling DW.

Checks whether the service callback constructor exists in isEnabled and
if not, disables DW service. This prevents crashing described in crbug/1375266.

Bug: 1375266
Change-Id: Ie2134ccbdd627b38b1b46e8ad4053227f2b765fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3960230
Reviewed-by: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Mahesh Machavolu <mahesh.ma@samsung.com>
Commit-Queue: Alex Mitra <alexmitra@chromium.org>
Reviewed-by: Peter Conn <peconn@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1060407}