Revert of Making the Mojo Channel Messages legacy mode dynamic. (patchset #4 id:120001 of )

Reason for revert:
Breaks the Chrome PFQ bot. Will need to sync with a change to the Arc++ code before landing this.

Original issue's description:
> Making the Mojo Channel Messages legacy mode dynamic.
> The mojo::edk::Channel::Message class uses a compile flag to determine
> whether to use the legacy or normal mode (legacy mode is used to keep
> backward compatibility between Arc++ and ChromeOS).
> Making that flag dynamically set on the embedder so that the default is
> now normal mode on Android. This is going to be used for adding
> Parcelable support on mojo Android (where we'll need the normal
> Message's header to include the parcelables).
> BUG=699311
> Review-Url:
> Cr-Commit-Position: refs/heads/master@{#457549}
> Committed:
# Not skipping CQ checks because original CL landed more than 1 days ago.

Cr-Commit-Position: refs/heads/master@{#458271}
4 files changed