Remove tentative from 3d-rendering-context-behavior test.

This removes the "tentative" indicator from the filename of
3d-rendering-context-behavior.tentative.html, since it's testing for a
behavior that is in the current spec.

The test is intended to test that both the 3D Rendering Context
extension caused by the transform-style property, and the effects of the
perspective property, apply based on parent-child element relationships
rather than containing block relationships.

In the case of perspective, this is stated informally in
https://drafts.csswg.org/css-transforms-2/#perspective :
  Second, the perspective and perspective-origin properties can be
  applied to an element to influence the rendering of its 3d-transformed
  children, ...
and more formally in the Accumulated 3D Transformation Matrix
Computation in
https://drafts.csswg.org/css-transforms-2/#accumulated-3d-transformation-matrix-computation

In the case of 3D Rendering Contexts, this is stated in
https://drafts.csswg.org/css-transforms-2/#3d-rendering-contexts which
says:
  Elements establish and participate in 3D rendering contexts as follows:

    * ...

    * An element participates in a 3D rendering context if its parent
      establishes or extends a 3D rendering context.

Bug: 1008483
Change-Id: I372db70b990bae21795db4569cdf1e5dd8e9c714
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2971464
Commit-Queue: David Baron <dbaron@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#894421}
diff --git a/css/css-transforms/3d-rendering-context-behavior.tentative.html b/css/css-transforms/3d-rendering-context-behavior.html
similarity index 100%
rename from css/css-transforms/3d-rendering-context-behavior.tentative.html
rename to css/css-transforms/3d-rendering-context-behavior.html