Give content_shell message headers include guards

This is in preparation for jumbo compiling content/shell.

IPC headers generate code by being included multiple times with
different macros. This requires careful juggling of inclusions
and jumbo is not careful with when headers are included. In most
of the code this has already been solved by adding include guards
but not in content/shell until this patch.

Bug: 886902
Change-Id: I84644fba628a827dfd7825d97844aacca3a9cd8e
Reviewed-on: https://chromium-review.googlesource.com/1233743
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592905}
5 files changed