[win] Fallback to registry when finding external handlers for a protocol

On windows 8 and later AssocQuery is used to find external protocol
handlers (e.g. to handle steam:// links). Prior versions use the
registry.

Some programs may not register in a way that AssocQuery recognizes, so
this change also consults the registry if AssocQuery does not return any
results.

BUG=560809

Review-Url: https://codereview.chromium.org/2172573002
Cr-Commit-Position: refs/heads/master@{#408322}
1 file changed