Remove obsolete seccomp_unwinder for legacy (pre-BPF) sandbox

Remove obsolete seccomp_unwinder for legacy (pre-BPF)  sandbox

The PopSeccompStackFrame was introduced to deal with stack frames
originated in the legacy seccomp sandbox. The only user of that
sandbox was Google Chrome, but the legacy sandbox has been
deprecated in 2013 (crrev.com/1290643003) in favor of the new
bpf sandbox.
Removing this dead code as it has some small bound checking bug
which causes occasional crashes in WebView (which are totally
unrelated to the sandbox).

Note: this will require a corresponding change in the chromium
GYP/GN build files to roll.

BUG=665, chromium:477444
R=jln@chromium.org, mark@chromium.org, torne@chromium.org

Committed: https://code.google.com/p/google-breakpad/source/detail?r=1492

[Primiano Tucci (use gerrit), 2015-08-17 11:03:15]

primiano@chromium.org changed reviewers:
+ mark@chromium.org, torne@chromium.org

[Primiano Tucci (use gerrit), 2015-08-17 11:04:26]

PTAL. Removing entire translation units is the best way to fix bugs :)

See
https://code.google.com/p/google-breakpad/issues/detail?id=665&q=PopSeccompSt...
for context.

[Torne, 2015-08-17 11:08:15]

lgtm

[Mark Mentovai, 2015-08-17 14:19:28]

mark@chromium.org changed reviewers:
+ jln@chromium.org

[Mark Mentovai, 2015-08-17 14:19:28]

jln to triple-check, but LGTM.

[jln (very slow on Chromium), 2015-08-17 17:06:41]

lgtm, +mdempsky FYI

[Primiano Tucci (use gerrit), 2015-08-18 08:38:23]

Committed patchset #2 (id:20001) manually as 1492 (presubmit successful).