Add UKM SourceID getter to WebContentsImpl.

Bug: 797586
Change-Id: I357e5b945886e51043d662973c9a1960f9b46929
Reviewed-on: https://chromium-review.googlesource.com/1086413
Commit-Queue: Steven Holte <[email protected]>
Reviewed-by: Alex Moshchuk <[email protected]>
Reviewed-by: Min Qin <[email protected]>
Cr-Commit-Position: refs/heads/master@{#569799}
14 files changed