VR: fix autofill popup size

Break SetAlertDialogSize into to methods. One to set the relative
size to the main content, another one to set the absolute size.
Also added comment to hosted_platform_ui to clarify the size of
the rectangle in the model.

Bug: 863071
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:linux_vr;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: Ie102fcb00865d66052e4bb3c7817831fdcfe5dfa
Reviewed-on: https://chromium-review.googlesource.com/1132140
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Amirhossein Simjour <asimjour@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574688}
4 files changed