Inline PDFiumEngine::InsetPage() and EnlargePage()

Remove these methods and inline them into their callers. Since the code
now uses gfx::Insets, they can be simplified by calling existing gfx
code instead of reinventing the wheel.

Change-Id: I317b1f4485b4d580c735ab80ab7dbc3515921807
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5618717
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Alan Screen <awscreen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313690}