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This fixes a regression where the platform impl was created even
when it wasn't yet needed.

Bug: 911184
Change-Id: I6555a81ff86327fd12994bf4de5e44074dfeee15
Reviewed-on: https://chromium-review.googlesource.com/c/1358919Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613207}