Split out separate BUILD.gn files from components/sync/BUILD.gn

This CL pulls out separate BUILD.gn files for those subfolders that
already have their own build targets:
base
device_info
driver
js
user_events

Bug: 947443
Change-Id: Icb9eb25dd3322f9ccfe2a6ecf320744725be4dad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1569224
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Auto-Submit: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653125}
15 files changed