Allow Mangler to be used outside of Inliner.

It now lives in the Compiler instead of the Inliner, and we have a
pointer to it inside the Context. I also took the opportunity to remove
a few unnecessary #includes from Context and fixed up the fallout from
this.

POTENTIALLY IMPORTANT FOOTNOTE: DSLWriter has its own Mangler object
inside of it. I experimented with removing this and having it share the
Mangler object inside the Context, but this caused failures in
"testThreading" and "testpersistentcache" CQ bots; the Expected and
Actual images would mismatch. (The Expected would be missing some
draws.) This particularly affected some blur-related tests which are
likely to be GaussianConvolution--i.e. it uses DSL.
This might be a canary in the coalmine for some quirky DSL threading
issue? e.g. using the same Context on two threads at once?

Change-Id: I2290b810d9487c40a3dbef119c95d1572b14a5ae
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/449357
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
12 files changed