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Makes it possible to enable LITTLE-cores-only mode via Finch, by
replacing the existing command line flag with a base::Feature.

It can still be enabled locally by passing
--enable-features=WebViewCpuAffinityRestrictToLittleCores or
enabling the feature in WebView DevTools.

Test: Run WV shell with flag above & check chrome://histograms/Power.
Bug: 1111789
Change-Id: I209b132ccd5354e41849a33bb0509773b619fc62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2502511
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821685}