Move SkMultiPictureDocument functions into namespace in include/docs

This "internal" file has been used by clients [1] [2], and doesn't expose
any private types, so we should properly make it available. This
is a requirement due to Bazel sandboxing and layer checking.

Rather than dumping functions into the global namespace, this adds
a SkMultiPictureDocument namespace with the necessary functions,
to align with our more modern design principles.

[1] https://source.chromium.org/chromium/chromium/src/+/main:printing/metafile_skia.cc;l=211;drc=610980924c853ea99afac1bc335509814a8fd540
[2] https://android.googlesource.com/platform/frameworks/base/+/0c175e28e5ca9b8056ab94c2281fa1354843e1c3/libs/hwui/pipeline/skia/SkiaPipeline.cpp#298

Change-Id: If05ed134d7b6be4c91dcc558022072a0e72553ce
Bug: b/40045064
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/764971
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
15 files changed