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- The legacy IPC pathway was checking the max data
size *before* the IPC call, and rejecting if it was
exceeded.

- This patch reinstates the check for the new mojo
pathway, so that we don't risk crashing the renderer
(thanks to verification in struct traits) if max data
is exceeded.

- This involved moving the max data constant to mojo
so it can be accessed from both blink and content.

R=peter@chromium.org

Bug: 798742,796991
Change-Id: I14ae782c9265411da1374beb7ae7312cb265b07e
Reviewed-on: https://chromium-review.googlesource.com/1011602Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Anita Woodruff <awdf@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551146}