[Mac] Set the nsexception crash key in TERMINATING_FROM_UNCAUGHT_NSEXCEPTION().

This was set previously back when the NSException initializers were swizzled,
and it can be used by the crash processor more easily than the
first/lastexception keys.

BUG=637270
R=mark@chromium.org

Review-Url: https://codereview.chromium.org/2561173002
Cr-Commit-Position: refs/heads/master@{#437350}
1 file changed