When not filled in, as in Android, activeGPUIndex is -1. This is fine
for Android because it doesn't use vendorID, but the function computed
vendorID unconditionally without checking for -1.

Default activeGPUIndex to 0 instead of -1. Note code still needs to
check for systemInfo.gpus.empty().

This caused crashes when _LIBCPP_DEBUG=0 was enabled.

Bug: chromium:923166
Change-Id: If4d1dff9553a580fd92bc0497fc092789d07ed93
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1925031Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>