views: add PlatformStyle docs

docs/ui/views/platform_style.md

+# Views Platform Styling
+
+## Overview
+
+Views controls may have different appearances on different platforms, so that
+Views UIs can fit better into the platform's native styling. This document
+describes how to build Views UIs that will look good on all platforms with a
+minimum of manual intervention.
+
+UIs looking good happens at two levels: first, the individual controls must look
+and act appropriately for their platform, and second, the overall layout of the
+controls in a dialog or UI surface must match what users of the platform would
+expect. There are differences at both of these layers between desktop platforms,
+and mobile platforms have still more differences.
+
+## Controls
+
+Individual controls have different looks and behaviors on different platforms.
+If you're adding a new control or a subclass of an existing control, there are
+some best practices you should follow in designing it so that it works well
+everywhere:
+
+### Use PlatformStyle for stylistic elements
+
+PlatformStyle exposes factory functions that produce different subclasses of
+Border, Background, and so on that are appropriate to the current platform. If
+your class needs a special kind of border or another stylistic element, creating
+it through a factory function in PlatformStyle will make per-platform styling
+for it easier, and will make which parts of the appearance are platform-specific
+more apparent. For example, if you were adding a Foo control that had a special
+FooBackground background, you might add a function to PlatformStyle:
+
+    unique_ptr<FooBackground> CreateFooBackground();
+
+and a default implementation in PlatformStyle. This way, in future a
+platform-specific implementation can go in PlatformStyleBar and change the
+background of that control on platform Bar without changing the implementation
+of the Foo control at all.
+
+### Use PlatformStyle to add simple behavior switches
+
+When adding platform-specific behavior for an existing control, if possible, it
+is useful to implement the switch using a const boolean exported from
+PlatformStyle, instead of ifdefs inside the control's implementation. For
+example, instead of:
+
+    #if defined(OS_BAR)
+    void Foo::DoThing() { ... }
+    #else
+    void Foo::DoThing() { ... }
+    #endif
+
+It's better to do this:
+
+    Foo::Foo() : does_thing_that_way_(PlatformStyle::kFooDoesThingThatWay)
+
+    void Foo::DoThing() {
+        if (does_thing_that_way_)
+            ...
+        else
+            ...
+    }
+
+This pattern makes it possible to unit-test all the different platform behaviors
+on one platform.
+
+### Use subclassing to add complex behavior switches

[sky, 2016/05/05 23:27:43]

Do you have an example of this? I could see this for focus/border/painting, but
in general we shouldn't subclass controls per platform. If we did, it makes
subclassing impossible.

[Elly Fong-Jones, 2016/05/06 19:23:29]

Done.

+
+If a lot of the behavior of Foo needs to change per-platform, creating
+platform-specific subclasses of Foo and a factory method on Foo that creates the
+appropriate subclass for the platform is easier to read and understand than
+having ifdefs or lots of control flow inside Foo to implement per-platform
+behavior.
+
+## UI Layout / Controls
+
+TODO(ellyjones): This section needs a bit more thought.
+
+Some platforms have conventions about the ordering of buttons in dialogs, or the
+presence or absence of certain common controls. For example, on Mac, dialogs are
+expected to have their "default" button at the bottom right, and expected not to
+have a "close" button in their top corner if they have a "Cancel"/"Dismiss"
+button in the dialog body. If you can design a layout that follows all
+platforms' conventions simultaneously, that is the lowest-effort route to
+follow, but if not, there are static booleans in PlatformStyle that hold the
+appropriate values for these decisions on the current platform, like:
+
+    static const bool PlatformStyle::kDialogsShouldHaveCloseButton;
+
+You can then condition your dialog creation code like this:
+
+    if (PlatformStyle::kDialogsShouldHaveCloseButton)
+        views::Button* close_button = ...;
+
+TODO(ellyjones): Actually add these variables to PlatformStyle