Fix missing affected histogram suffix for Sync.E2ELatency

Follow-up to https://chromium-review.googlesource.com/c/1292567 where
the metric was instrumented, and where the relation between the new
histogram and the suffix was forgotten.

Bug: 902343
Change-Id: I1a9de66f522ebcdc70ded3cb3449e1b1783a6a98
Reviewed-on: https://chromium-review.googlesource.com/c/1370165
Reviewed-by: Tatiana Gornak <melandory@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615241}