The code is easier to understand when it's encapsulated better.

This change is pure refactoring with no functional changes.

BUG=angleproject:1442
TEST=angle_end2end_tests

Change-Id: I6128fd72c65ca7a87af596cda4866c74c2a66c48
Reviewed-on: https://chromium-review.googlesource.com/452502
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>