In the shadow DOM v1 it is impossible to set styles for nested shadow-dom
elements. To fix the problem, style was added directly to gr-diff
element as a style module (gr-syntax-layer does it in the same way).
It is impossible to fix this problem with ::slotted, because ::slotted
can't select a descendant of a top-level distributed child.
Bug: Issue 11323
Change-Id: I4fcab47a40255490cd922f921502a8532dbffcf8
9b4836aa07