Previously the tool would save to a separate file for every frame and
one additional file for the setup function. Frames without binary data
would not get an "angledata" binary file.

Using a single file has a few advantages:

 - makes it simpler to track the data dependencies in GN
 - should be easier to compress and decompress for speed up transfers
 - no need to wonder if we have frames with missing data files
 - less overhead reading from the HDD since the whole trace is buffered

We might need to worry about extemely huge memory use cases. For now
buffering the whole file into memory on startup is simple and fast.

Also makes the binary data loader function more robust.

Bug: angleproject:4484
Change-Id: Ia74fb0bc65200adf7a21c3143f98aa4da86457b1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2104554
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>