Use "mojo-message-error" instead of "bad_message_reason" crash key name.

NetworkService was incorrectly using "bad_message_reason" crash key name
when reporting a bad mojo message.  This CL switches to using the
correct "mojo-message-error" name (one that is recognized by the crash
analysis services).  This CL also moves the crash key definition to the
//mojo layer, so that it can be reused both from //content and from
//services/network.

Bug: 987986
Change-Id: I772ae563df2f62ea430235524397b8492455de23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2118690
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Maksim Orlovich <morlovich@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753484}