PlzNavigate: Add helper method for checking if PlzNavigate is enabled.

In the interest of simplifying the code for checking if browser side
navigation is enabled this change creates a new helper method and
replaces all current occurrences of the "extended" form.

BUG=368813
TBR=mmenke@chromium.org

Review URL: https://codereview.chromium.org/1517643002

Cr-Commit-Position: refs/heads/master@{#365527}
35 files changed