This change decouples gr-diff-builder and gr-syntax-layer. gr-syntax-
layer will now live in gr-diff-host.
As a result, from the perspective of gr-diff/gr-diff-builder, plugin
layers and the syntax layer will now be part of the same list.
(This change fixes a time reporting regression in https://gerrit-review.googlesource.com/c/gerrit/+/239972.)
A future potential refactor could also lift gr-coverage-layer instances
in a similar fashion.
Change-Id: I47e91fdac00e17460aab9ca57e4569ade63c2eeb