This CL enables the hide panel functionality
upon uploading a new file. File reader emits
fileuploadstart event when it receives a new file
which hides the panels.

Bug: v8:10644

Change-Id: Ic26cce1a92559efd494f2ef1e32b514897a73324
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2305892
Commit-Queue: Zeynep Cankara <zcankara@google.com>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69015}