Make Mac PrintRenderFrameHelper::RenderPage() less Mac-specific.

Rename it to PrintPageInternal() and give it the same method signature
as PrintPageInternal() on other platforms. Change both
PrintPageInternal() implementations to look more like each other.

Change-Id: I087f784eeea8bf1b9dc4e5e0c8b2d1d30572d204
Reviewed-on: https://chromium-review.googlesource.com/780686


Reviewed-by: Wei Li <weili@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518318}