Forbid implicit conversions from IntRect to FloatRect

This is a followup from removing some accidental conversions via FloatRect. Since
FloatRect is not a perfect superset of IntRect, converting from IntRect to FloatRect is
potentially lossy, and conversions that happen by accident hide that APIs are not used
as intended.

A few new helper functions that use IntRect were added to make the change smaller, but
there is potential for future further cleaning up of what types are used where, and how.

Behaviour has been kept where it's not obviously a mistake. Truncating or snapping to
integers is sometimes beneficial.

R=fs@opera.com, pdr@chromium.org

Bug: 860535

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_layout_tests_layout_ng;luci.chromium.try:linux_layout_tests_slimming_paint_v2;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I733b4273b847654f22bae726255cfb75ba8023bb
Reviewed-on: https://chromium-review.googlesource.com/1124454
Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#573263}
26 files changed