In a pattern like this:

- allocate 100 bytes
- allocate 200 bytes
- allocate 50 bytes
- allocate 100000000 bytes
- allocate 100 bytes
- allocate 200 bytes
- allocate 50 bytes

The DynamicBuffer class switches to making 100MB allocations even if
that allocation was a one-off.  A small future allocation would then tie
up 100MB in memory for future allocations.  Another 100MB is also left
tied up in the free list.

With this change, if an allocation is made that's less than a quarter of
the DynamicBuffer's current allocation size, it's taken as a sign that
the previous large allocation was a one-off and the size is moved back
to the DynamicBuffer's original initial size.

Bug: b/187757166
Bug: chromium:1224952
Change-Id: Icb69bfa3196daa1ee8e6c38ef9513730f9afacfa
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2991915Reviewed-by: Jamie Madill <jmadill@chromium.org>