The only usage of `virtual` was the `storage()` method. This value is
now stored in a member variable. It is packed next to a bool so it
should be ~zero extra space, versus 8 bytes for a vtable pointer. We
also save cost of a virtual dtor call every time a DSLVar goes away.
Because VariableStorage is not a public type, this required shuffling
some constructors around to live in the cpp instead of the header.
Change-Id: I9fdefc3696d123848fb567029c051b478349cec7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/542139
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
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