Remove WebContents from OmniboxEditController::Update()
The one caller of this method shared with iOS calls this method with NULL, so
change the method to have the semantics of being called with NULL and have the
variants that take in a WebContents be defined in the //chrome-level
subclasses.
As always, the motivation is to remove blockers to sharing this code with iOS,
which cannot use //content.
BUG=511944
Review URL: https://codereview.chromium.org/
1252873003
Cr-Commit-Position: refs/heads/master@{#340689}