Revert of Revert of Initial draft - modify ViewportAnchor to know about both inner and outer viewports. (patchset #1 id:1 of https://codereview.chromium.org/597113002/)

Reason for revert:
Reapplying, reverting this one shouldn't be necessary.

Original issue's description:
> Revert of Initial draft - modify ViewportAnchor to know about both inner and outer viewports. (patchset #11 id:200001 of https://codereview.chromium.org/556703005/)
> 
> Reason for revert:
> The new test is failing on mac. Reverting for now so it can be investigated.
> 
> [  FAILED  ] PinchViewportTest.TestResizeAfterHorizontalScroll (39 ms)
> [1471/1471] PinchViewportTest.TestResizeAfterHorizontalScroll (39 ms)
> Retrying 2 tests (retry #3)
> [ RUN      ] PinchViewportTest.TestResizeAfterVerticalScroll
> ../../third_party/WebKit/Source/web/tests/PinchViewportTest.cpp:251: Failure
> Value of: (pinchViewport.visibleRect().size()).width()
> Actual: 45.94595
> Expected: (FloatSize(50, 25)).width()
> Which is: 50
> ../../third_party/WebKit/Source/web/tests/PinchViewportTest.cpp:251: Failure
> Value of: (pinchViewport.visibleRect().size()).height()
> Actual: 22.972975
> Expected: (FloatSize(50, 25)).height()
> Which is: 25
> ../../third_party/WebKit/Source/web/tests/PinchViewportTest.cpp:253: Failure
> Value of: (frame()->view()->scrollPosition()).y()
> Actual: 638
> Expected: (IntPoint(0, 625)).y()
> Which is: 625
> ../../third_party/WebKit/Source/web/tests/PinchViewportTest.cpp:254: Failure
> Value of: (pinchViewport.location()).y()
> Actual: 62
> Expected: (FloatPoint(0, 75)).y()
> Which is: 75
> 
> Original issue's description:
> > Fix pinch virtual viewport position after resize.
> > 
> > Associate the viewport anchor with the inner viewport.
> > Adjust the inner and outer viewport positions after resize
> > such that they both remain in their allowed range and the
> > inner viewport origin scales proportionally within the
> > outer viewport.
> > 
> > As a small cleanup, made the method ScrollView::scrollTo() protected.
> > 
> > BUG=364108
> > 
> > Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=182365
> 
> TBR=leviw@chromium.org,aelias@chromium.org,bokan@chromium.org,timav@google.com,timav@chromium.org
> NOTREECHECKS=true
> NOTRY=true
> BUG=364108
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=182580

TBR=leviw@chromium.org,aelias@chromium.org,bokan@chromium.org,timav@google.com,timav@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=364108

Review URL: https://codereview.chromium.org/597683007

git-svn-id: svn://svn.chromium.org/blink/trunk@182586 bbb929c8-8fbe-4397-9dbb-9b2b20218538