Use in-class initializers for base::WeakPtrFactory (all remaining uses on Linux desktop).

Changed NULL to nullptr where the presubmit complained (due to new line wrapping).

Generated by: https://chromium-review.googlesource.com/c/chromium/src/+/1689216

Bug: 981415
Tbr: johannes@chromium.org,mathp@chromium.org,satorux@chromium.org
Change-Id: Icd574b69f75ead6dc6f34e7b4edcd52a53830640
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1702744


Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Raymes Khoury <raymes@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Yuwei Huang <yuweih@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#686003}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 7c5cfabd6bc582c20e0ae5e56adf8e9c8e22bce0