[BRP] Rename CheckedPtr<T> to raw_ptr<T>

The trickiest part of the CL was that variables of type raw_ptr<T> were
renamed to wrapped_ptr, instead of raw_ptr, to disambiguate from
variables of T* which is an actual raw pointer.

Improved comments and docs slightly, while at it.

Bug: 1073933
Change-Id: I5bac0a46758b0668bfa33931718def6382f79fd7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3015914


Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Adam Langley <agl@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#906338}