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This CL adds two new ways to specify edge case behavior for FlexLayout
that will make some use cases much easier.

1. SetTargetArea(LayoutTargetArea) - specifies whether the host view's
insets are treated as inviolable (default and older behavior) or whether
they're considered part of the internal margins of the host view for
layout purposes. The new (optional) behavior allows certain cases where
a child view with internal padding cannot currently get close enough to
a host view border, inkdrop, or focus ring.

2. Set[Main|Cross]AxisMarginsPolicy(LayoutMarginPolicy) - specifies
whether child margins apply when they abut the edge of the host view.
The new (optional) behavior allows much simpler code when spacing between
views needs to be different than spacing at the edges of the host view.

OPEN QUESTIONS:
 - Should we just make the new behavior for (1) the default?

Bug: 898632
Change-Id: I5d336ed19b32a791f944f8ced615a099a0729802
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1676736
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683293}