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We plan to start vulkan finch for Linux Dev. This change labels UMA
data with backend names (GL, VK, etc). So we can compare metrics in UMA
by labels.

Bug: 1022495,1012393
Change-Id: I75404b4c7fc8d63d09e145b7a03342a7439e88ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1924632Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Jonathan Backer <backer@chromium.org>
Commit-Queue: Peng Huang <penghuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718118}