https://bugs.webkit.org/show_bug.cgi?id=93898
<rdar://problem/12089098>

Reviewed by Simon Fraser.

Add logging when we enter and exit the threaded scrolling mode, and logs the reasons we
fall into main-thread scrolling.

The logging output looks like this:
    SCROLLING: Switching to main-thread scrolling mode. Time: 15843.554718 Reason(s): viewport-constrained objects
    SCROLLING: Switching to threaded scrolling mode. Time: 15844.550866
    SCROLLING: Switching to main-thread scrolling mode. Time: 15845.551214 Reason(s): viewport-constrained objects
    SCROLLING: Switching to threaded scrolling mode. Time: 15846.552619
    SCROLLING: Switching to main-thread scrolling mode. Time: 15847.553587 Reason(s): viewport-constrained objects
    SCROLLING: Switching to threaded scrolling mode. Time: 15848.554084

No new tests, as this is just debugging logging.

* page/scrolling/ScrollingCoordinator.cpp:
(WebCore::ScrollingCoordinator::updateShouldUpdateScrollLayerPositionOnMainThread):
Construct a bitfield describing the reasons we fall into main-thread scrolling mode.

* page/scrolling/ScrollingCoordinatorNone.cpp:
(WebCore::ScrollingCoordinator::setShouldUpdateScrollLayerPositionOnMainThread):
* page/scrolling/ScrollingTreeNode.cpp:
(WebCore::ScrollingTreeNode::ScrollingTreeNode):
* page/scrolling/ScrollingTreeNode.h:
(WebCore::ScrollingTreeNode::shouldUpdateScrollLayerPositionOnMainThread):
* page/scrolling/ScrollingTreeState.cpp:
(WebCore::ScrollingTreeState::ScrollingTreeState):
(WebCore::ScrollingTreeState::setShouldUpdateScrollLayerPositionOnMainThread):
* page/scrolling/ScrollingTreeState.h:
(WebCore::ScrollingTreeState::shouldUpdateScrollLayerPositionOnMainThread):
* page/scrolling/chromium/ScrollingCoordinatorChromium.cpp:
(WebCore::ScrollingCoordinator::setShouldUpdateScrollLayerPositionOnMainThread):
* page/scrolling/ScrollingCoordinator.cpp:
(WebCore::ScrollingCoordinator::setShouldUpdateScrollLayerPositionOnMainThread):
Use the reasons bitfield instead of a boolean.

* page/scrolling/ScrollingCoordinator.h:
(ScrollingCoordinator):
Add MainThreadScrollingReasons enum, with the current reasons that we might fallback to main-thread scrolling.

* page/scrolling/ScrollingTree.cpp:
(WebCore::ScrollingTree::scrollingPerformanceLoggingEnabled):
Fix a typo (scrollingPeformanceLoggingEnabled -> scrollingPerformanceLoggingEnabled).

* page/scrolling/mac/ScrollingTreeNodeMac.mm:
(WebCore::ScrollingTreeNodeMac::update):
(WebCore::ScrollingTreeNodeMac::setScrollPosition):
(WebCore::logThreadedScrollingMode):
Pretty-print the scrolling mode and shouldUpdateScrollLayerPositionOnMainThreadReason.

Update the scrolling tree's scrollingPerformanceLoggingEnabled preference
before the early-return if we don't have layer debugging borders on.

* WebProcess/WebPage/mac/TiledCoreAnimationDrawingArea.mm:
(WebKit::TiledCoreAnimationDrawingArea::updatePreferences):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@128521 268f45cc-cd09-0410-ab3c-d52691b4dbfc