Triage and fix most dangling pointers hit by blink_platform_unittests

In most files, the fix is just a reordering of member declarations.
In a few files, the member variables were initialized at declaration
time, which had to be changed to explicit initialization in the ctor.

The remaining dangling pointers under the blink_platform_unittests will
be addressed in a separate CL since the fixes are non-trivial.

Validated the fixes by running blink_platform_unittests with dangle
detection enabled.

Bug: 40212619
Change-Id: I1f21b062da4971fcd2ecae879c3d1d1431732acc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5420069


Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Miguel Casas-Sanchez <mcasas@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Auto-Submit: Abhishek Shanthkumar <abhishek.shanthkumar@microsoft.com>
Reviewed-by: John Rummell <jrummell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282146}