[vm] Tweak location of Perfetto protos

This CL tries to address version skew issues which occur in internal
monorepo which we created by checking in *.pbzero.h files into
runtime/vm/protos/*. These files refer to protozero internal headers,
which in the internal monorepo come from a shared Perfetto dependency
which does not necessarily match the version we pinned in our DEPS and
used to generated these files.

This version skew caused problems for the Perfetto team, see
https://github.com/google/perfetto/pull/3195

To resolve this change layout of our repo to allow us to depend on the
shared Perfetto entirely - including its version of *.pbzero.h headers.

We change include paths in the VM source to

    #include "third_party/perfetto/protos/..."

which match location of these headers in the internal monorepo.

To make the code compile in the normal SDK checkout we shift location of
Perfetto and checked in code:

* Perfetto source moves from third_party/perfetto to
  third_party/perfetto/src.
* Generated code moves from runtime/vm/protos into
  third_party/perfetto/protos.

Note: experiments show that *.pbzero.h files tree-shake very well and
you pay only for things that you use. That means depending on *.pbzero.h
files generated from full message definitions rather than our manually
tree-shaken versions does not actually increase the size of the VM
binaries. We could switch our external build to do the same but this
would make protoc a build time dependency, which seems excessive.

TEST=ci

Change-Id: I724b9d9304be83302a5d83c2337154b36a6a6964
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/454260
Commit-Queue: Slava Egorov <vegorov@google.com>
Reviewed-by: Ben Konyi <bkonyi@google.com>