Simplify calulations of paddingBox and borderBox in InspectorOverlay.cpp
Simplify calulations of paddingBox and borderBox in InspectorOverlay.cpp

We can simplify equations calculating paddingBox and borderBox. For instance...
- clientBoxWidth = frameRectWidth - borderLeft - borderRight - verticalScrollbarWidth.
- contentBoxWidth = clientBoxWidth - paddingLeft - paddingRight + verticalScrollbarWidth

Thus..
- paddingBoxWidth = contentBoxWidth + paddingLeft + paddingRight => clientBoxWidth + verticalScrollbarWidth
- borderWidth = paddingBoxWidth + borderLeft + borderRight
  => clientBoxWidth + verticalScrollbarWidth + borderLeft + borderRight
  => frameRectWidth.

BUG=432192

Review URL: https://codereview.chromium.org/865303002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@188966 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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