[AppCache] ReportBadMessage rather than DCHECK for preconditions on mojo API.

Nothing stops a renderer from calling these methods while another call
is still processing, so ReportBadMessage rather than DCHECK when that
happens.

Bug: 922398
Change-Id: I2ab0abf245828eaa81ce8fbfbc7396db03029d86
Reviewed-on: https://chromium-review.googlesource.com/c/1416092
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623462}
5 files changed