[LayoutNG] Support fragments in InspectorCSSAgent

This patch implements LayoutNG inline fragment support for
InspectorCSSAgent::CollectPlatformFontsForLayoutObject(). This
function is used for Inspector to show rendered fonts. Also some font
fallback tests rely on this mechanism.

Bug: 636993
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I7e43fe1b50eef831fb77f6e4a4b2dc59b34569e9
Reviewed-on: https://chromium-review.googlesource.com/1003016
Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Commit-Queue: Dominik Röttsches <drott@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549826}
7 files changed