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Most of our Views class properties are of type ClassProperty<T*> where
T is a value type, but we're forced to use pointers because T can be of
any type and must therefore be allocated on the heap.

This leads to a lot of code like:
my_view->SetProperty(kMarginsKey, new Insets(kMyViewDefaultInsets));
...
*my_view->GetProperty(kMarginsKey) = new_insets;

(The latter pattern is to prevent a second heap allocation - but only
works if the initial allocation happens; otherwise it crashes.)

This CL shortcuts this behavior so that it behaves in the way we
actually want to use the system 90% of the time:

// Allocates a copy of kMyViewDefaultInsets for the property.
my_view->SetProperty(kMarginsKey, kMyViewDefaultInsets);

// Updates the value of the existing property.
my_view->SetProperty(kMarginsKey, new_insets);

// De-allocates the existing property value.
my_view->ClearProperty(kMarginsKey);

In order to use this new functionality:
 - The property must be an owned property of pointer type.
 - The property type behind the pointer must be copy- or
   move-assignable.

Change-Id: Idec1d5b9c104814f234270214b615774fa5a7084
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1657288Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670329}