Lift gr-syntax-layer into gr-diff-host

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
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
index 32fa7e5..374368c 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -378,7 +378,7 @@
               line-wrapping="[[lineWrapping]]"
               is-image-diff="[[isImageDiff]]"
               base-image="[[baseImage]]"
-              plugin-layers="[[pluginLayers]]"
+              layers="[[layers]]"
               revision-image="[[revisionImage]]">
             <table
                 id="diffTable"