Make offset from ancestor computation explicit by moving it to the callers.
https://bugs.webkit.org/show_bug.cgi?id=134168.

Reviewed by Simon Fraser.

Explicit offset computation helps code reading and less error-prone than passing
the offset pointer and compute it locally when it is null.

No change in functionality.

* rendering/RenderLayer.cpp:
(WebCore::transparencyClipBox):
(WebCore::performOverlapTests):
(WebCore::RenderLayer::setupClipPath):
(WebCore::RenderLayer::setupFilters):
(WebCore::RenderLayer::paintLayerContents):
(WebCore::RenderLayer::collectFragments):
(WebCore::RenderLayer::updatePaintingInfoForFragments):
(WebCore::RenderLayer::paintTransformedLayerIntoFragments):
(WebCore::RenderLayer::hitTestLayer):
(WebCore::RenderLayer::hitTestTransformedLayerInFragments):
(WebCore::RenderLayer::calculateClipRects):
* rendering/RenderLayer.h:
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::calculateCompositedBounds):
(WebCore::RenderLayerCompositor::requiresCompositingForPosition):
* rendering/RenderTreeAsText.cpp:
(WebCore::writeLayers):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@170282 268f45cc-cd09-0410-ab3c-d52691b4dbfc