Handle ERR_PAC_SCRIPT_TERMINATED in ProxyService.
If the out-of-process v8 pac resolver crashes, ProxyResolverMojo
returns ERR_PAC_SCRIPT_TERMINATED as its error. This changes
ProxyService to discard its ProxyResolver when that error is returned,
and if other requests were in progress, start reinitializing a new
ProxyResolver.
This also changes ProxyResolverMojo to correctly handle being deleted
while running a CompletionCallback.
BUG=467832
Review URL: https://codereview.chromium.org/
1069483003
Cr-Commit-Position: refs/heads/master@{#326960}