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This change prepares all public methods of PpdProvider to accept
arguments that may contain mixed-case effective-make-and-model strings
and explicitly converts any such members to lowercased ASCII.

Bug: chromium:888189
Test: none
Change-Id: I7ed8399858924828c138f0c27f86408a9ddcae32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363656
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Reviewed-by: Luum Habtemariam <luum@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805954}