Reland: 'Fix Performance::now() for browser navigations'.

Previously this was reverted because it was introducing an assert that
doesn't hold in some valid cases. This time we're skipping this assert.

Original review: https://codereview.chromium.org/379873002/
More details:
https://code.google.com/p/chromium/issues/detail?id=395817#c10

Original commit message:

Performance::now() is defined 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 the
records.

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

BUG=376004

Review URL: https://codereview.chromium.org/417513002

git-svn-id: svn://svn.chromium.org/blink/trunk@178782 bbb929c8-8fbe-4397-9dbb-9b2b20218538