aw: Fix multiprocess flag usage

Stop using kSingleProcess, which does not mean what we think it means.
Pass the correct webview process type to breakpad, so that microdumps
contain the correct information.

BUG=708773

Review-Url: https://codereview.chromium.org/2806733002
Cr-Commit-Position: refs/heads/master@{#463302}
6 files changed