Removal of base::Time::ToInternalValue() from kSAMLLastGAIASignInTime.

Deprecated ToInternalValue() and FromInternvalValue() are being removed
from pref::kSAMLLastGAIASignInTime. This CL is submitted in preparation
for online sync on the lock screen feature.

Bug: 634507
Change-Id: If9afee3be3773b9a929940a82c361d635da988d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2245136
Commit-Queue: Maciek Slusarczyk <mslus@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#778864}