When switching log files, if a function was selected the timeline would
attempt to draw its ticks and would usually crash. It would also
preserve the selected section of the timeline.

This patch wipes out the current state when loading a new log file,
with the exception of the current mode (Summary, Bottom up, etc). As a
consequence, the timeline size is now calculated in TimelineView.render
so it will always be drawn at the right size.

Bug: v8:6240
Change-Id: Ie15dd9b225901c8f4df614444458ae2aeaf74ccc
Reviewed-on: https://chromium-review.googlesource.com/1186340Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55339}