This CL adds a new way to set the start frame of a capture.
It adds a new environment variable called ANGLE_CAPTURE_TRIGGER
that, when set, will be used instead of frame start and end.

By setting ANGLE_CAPTURE_TRIGGER to a non-zero value, ANGLE will
capture that many frames when the value changes. For example,
on Android, we can set it with:

  adb shell setprop debug.angle.capture.trigger 20

When we reach the target content, set the value back to zero:

  adb shell setprop debug.angle.capture.trigger 0

and ANGLE will start capturing 20 frames.

Currently only hooked up for Android, but should be possible to
support on other platforms.

Test: Capture application frames using trigger
Bug: angleproject:4949
Change-Id: I469ef5c48feb78c85b8cda2fefd5df59e495bbe2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2347858
Commit-Queue: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Manh Nguyen <nguyenmh@google.com>