Pass base::Value by value to SetExtensionPref

Change ExtensionPrefValueMap::SetExtensionPref to take the
base::Value parameter by value instead as std::unique_ptr<>.

Fix client code to use the new API.

Bug: 646113
Change-Id: I127b361ea7f44c9e2a753c113adedac90bc9d5bc
Reviewed-on: https://chromium-review.googlesource.com/c/1448162
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628334}
10 files changed