Replace if () NOTREACHED() with CHECK() in ['ppapi/proxy', 'ppapi']

Replace simple cases of if (foo) NOTREACHED(); and if (!bar)
NOTREACHED() with CHECK(!foo); and CHECK(bar) in
['ppapi/proxy', 'ppapi'].

This aligns with current guidance:
https://chromium.googlesource.com/chromium/src/+/main/styleguide/c++/checks.md#:~:text=Use%20CHECK(bar)%3B%20instead

Most changes were automated with regular expression substitutions.

In some cases the expression in the condition could have side effects.
In these cases, a temporary variable was manually added to hold the
return value and then CHECK()d separately, ensuring that the side-effect
still happens if the CHECK is removed.

This CL was uploaded by git cl split.

R=dschuff@chromium.org

Change-Id: If760799c4793fddfb3395271b5111b66e70658f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5831667
Reviewed-by: Derek Schuff <dschuff@chromium.org>
Commit-Queue: Derek Schuff <dschuff@chromium.org>
Auto-Submit: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1350203}