Extensions: Move IsIncognitoEnabled to extensions/ from chrome/.

This CL moves extensions::util::IsIncognitoEnabled from
chrome/browser/extension_util.h(cc) to extensions/browser/extension_util.h(cc).
This is possible since IsIncognitoEnabled does not have any chrome/
dependencies. This is required to prevent a dependency violation for a
subsequent CL where we need to use IsIncognitoEnabled from code in
src/extensions.

BUG=527548

Review-Url: https://codereview.chromium.org/2758103003
Cr-Commit-Position: refs/heads/master@{#462274}
28 files changed