Use ANGLE + SwiftShader when running under rr debugger.

As discussed in https://github.com/rr-debugger/rr/issues/3341 , some
configurations of running Chromium or ChromeOS can end up with graphics
drivers that write directly to the process memory.  Using ANGLE +
SwiftShader appears to be a good way to avoid this.

This should make running Chromium under rr a more reliable experience
out-of-the-box, although it does mean that it's more clearly not usable
for parts of the graphics stack that, at least in many cases, aren't
usable under rr anyway.

Bug: 1359355
Change-Id: I7d779cf0383f9fe9ea2bebff8543909c282a68ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3787592


Commit-Queue: David Baron <dbaron@chromium.org>
Reviewed-by: Alexis Hétu <sugoi@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1044516}