Allow Registry Iterator functions to use a specified WOW64 mode when iterating.

Also fix up any callers to access the correct hive (32 or 64) depending on
where the data is, or whether the hive is Redirected, Shared, or Reflected
as specified in MSDN

http://msdn.microsoft.com/en-us/library/windows/desktop/aa384253.aspx

BUG=338706,408018
TEST=Follow steps in 408018 and make sure extension gets added on 64-bit Chrome
TEST=base_unittests --gtest_filter=Registry*

Review URL: https://codereview.chromium.org/616173003

Cr-Commit-Position: refs/heads/master@{#298758}
7 files changed