DevTools: add Page.frameRequestedNavigation, deprecate Page.frameScheduledNavigation and frameClearedScheduledNavigation

- add FrameRequestedNavigation probe that gets dispatched whenever client-initiated navigation request is sent to browser and carries reason for navigation;
- expose the above as Page.frameRequestedNavigation
- deprecate Page.frameScheduledNavigation and Page.frameClearedScheduledNavigation;


Change-Id: Ia652b5381c22b68962a6f568e7cb18456dc0269a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1524706
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642246}
16 files changed