DevTools: NetworkPanel: improve XHR resource type detection.

1) Make decision on the base of initiatorInfo, because Resource::Raw type is too ambiguous.
2) Preserve discovered XHR resource type for the whole request lifecycle.

BUG=234025

Review URL: https://codereview.chromium.org/671423002

git-svn-id: svn://svn.chromium.org/blink/trunk@184636 bbb929c8-8fbe-4397-9dbb-9b2b20218538