Refactor Skia document type into an enum class

Change Skia document type from an enum type to an enum class type so
that type checking will be stricter and more clear.

BUG=455764
TBR=eseckler@chromium.org

Change-Id: Ia474e0c0a2b607884dc2f248301f3b003ef07e66
Reviewed-on: https://chromium-review.googlesource.com/619502
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496202}

• chrome/browser/printing/print_view_manager_base.cc[diff]
• components/printing/renderer/print_render_frame_helper.cc[diff]
• components/printing/renderer/print_render_frame_helper_linux.cc[diff]
• components/printing/renderer/print_render_frame_helper_mac.mm[diff]
• components/printing/renderer/print_render_frame_helper_pdf_win.cc[diff]
• components/printing/test/mock_printer.cc[diff]
• headless/lib/browser/headless_print_manager.cc[diff]
• printing/common/pdf_metafile_utils.h[diff]
• printing/pdf_metafile_skia.cc[diff]