Fix Performance::now() for browser navigations.

Performance::now() is defined[1] as number of milliseconds since
navigationStart. It works correctly for renderer-initiated navigations,
but for browser-initiated ones we override the navigationStart with the
recording taken on the browser side and this is not represented in
Performace::now().

Among other reasons to be sad about this, this bug makes Chromium's
pagecyclers not represent the renderer initialization times in their
plt measurements.

This patch fixes this by adjusting the fields that are supposed to
correspond to navigationStart when we change navigationStart from the
embedder.

[1] http://www.w3.org/TR/hr-time/#dom-performance-now

BUG=376004

Review URL: https://codereview.chromium.org/379873002

git-svn-id: svn://svn.chromium.org/blink/trunk@178558 bbb929c8-8fbe-4397-9dbb-9b2b20218538