OnionSoup: Move copyless_paste.mojom to public/mojom/document_metadata

This CL moves document_metadata from public/platform/modules to
public/mojom to gather mojom files to mojom folder and updates
paths for copyless_paste.mojom.mojom.

Bug: 919393
Change-Id: I7b37ec38c24ae09f86106e75a3b920836688d78f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1535689
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644571}