Moves initialization of exception handler registration

Moves initialization of exception handler registration

Registering exception handlers is per-process, not per thread. The old
code was doing the registration per mojo-app, which is per
thread. This is problematic for a couple of reasons, but primarily
because an app may be unloaded, which results in the system having an
exception handler pointing to a now invalid location.

The fix is to move registration to the place where we launch
applications, which happens only once per process.

BUG=526245
TEST=none
R=yzshen1@chromium.org

Review URL: https://codereview.chromium.org/1317333003

Cr-Commit-Position: refs/heads/master@{#346240}