Lose/restore WebGL contexts if multisampling blackist status changes at runtime.

Source/core/page/Settings.in

 # FIXME: Add support for global settings.
 # FIXME: Add support for custom getters/setters.
 
 defaultTextEncodingName type=String
 editableLinkBehavior type=EditableLinkBehavior, initial=EditableLinkDefaultBehavior
 textDirectionSubmenuInclusionBehavior type=TextDirectionSubmenuInclusionBehavior, initial=TextDirectionSubmenuAutomaticallyIncluded
 passwordEchoDurationInSeconds type=double, initial=1
 
 # Sets the magnification value for validation message timer.  If the
 # magnification value is N, a validation message disappears automatically after
@@ skipping 58 matching lines @@
 acceleratedCompositingForAnimationEnabled initial=true
 acceleratedCompositingForFixedPositionEnabled initial=false
 acceleratedCompositingForOverflowScrollEnabled initial=false
 
 # Works only in conjunction with forceCompositingMode.
 acceleratedCompositingForScrollableFramesEnabled initial=false
 compositedScrollingForFramesEnabled initial=false
 
 webGLEnabled initial=false
 webGLErrorsToConsoleEnabled initial=true
-openGLMultisamplingEnabled initial=true

[abarth-chromium, 2013/05/06 15:19:20]

This CL is going in the wrong direction.  We want to autogenerate more of the
Settings object over time, rather than less.  If you need a notification pathway
for this data, I'd recommend moving this data to another objects.  Settings is
just a passive store of data.

[Ken Russell (switch to Gerrit), 2013/05/06 17:43:16]

We don't want to invent a completely new way of passing down this single bit of
data. There is already extensive support for updating Settings on the fly via
WebPreferences and ViewMsg_UpdateWebPreferences. It is used by the Settings UI
among other things. There are already hooks in Settings.cpp which notify the
Page of various changes, such as image loading, changes to the font family,
etc., which may trigger style recalc and other things in response.

Talking with dglazkov, I think a good direction would be to add a more generic
observer to watch for updates to the page's Settings. Then this setting would
not need to be handwritten. What do you think? If you think this is not a good
direction, then please propose an alternative. I don't think cloning the entire
Settings/WebPreferences infrastructure is palatable.

 privilegedWebGLExtensionsEnabled initial=false
 accelerated2dCanvasEnabled initial=false
 antialiased2dCanvasEnabled initial=true
 webAudioEnabled initial=false
 fullScreenEnabled initial=false
 asynchronousSpellCheckingEnabled initial=false
 memoryInfoEnabled initial=false
 
 # This feature requires an implementation of ValidationMessageClient.
 interactiveFormValidationEnabled initial=false
@@ skipping 42 matching lines @@
 # Some apps could have a default video poster if it is not set.
 defaultVideoPosterURL type=String
 
 smartInsertDeleteEnabled initial=defaultSmartInsertDeleteEnabled
 selectTrailingWhitespaceEnabled initial=defaultSelectTrailingWhitespaceEnabled
 
 selectionIncludesAltImageText initial=false
 
 # This setting enables touch based text selection and editing.
 touchEditingEnabled initial=false