Remove WebURLRequest::initialize() and simplify WebURLRequest

Does the same as we did for WebURLResponse in https://crrev.com/2117313002 and
https://crrev.com/2133693002.

BUG=625529
TBR=jochen@chromium.org for components

Review-Url: https://codereview.chromium.org/2140523002
Cr-Commit-Position: refs/heads/master@{#404894}
23 files changed