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It really should be called PrintHostMsg_DidStartPreview. Getting the
page count is usually one of the first steps in starting a print
preview, but the IPC message does a bit more than that now.

Change-Id: I001a66c169fa5cf42b238605d6067bf12d58962e
Reviewed-on: https://chromium-review.googlesource.com/1184317Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585298}