Have ScopedStyleResolver deal with ContainerNode references, not pointers

Have ScopedStyleResolver deal with ContainerNode references, not pointers as
it can never be NULL. This is now obvious to callers that the ContainerNode
cannot be NULL.

BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23567016

git-svn-id: svn://svn.chromium.org/blink/trunk@157090 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Recompute percentage based @viewport on resize.

Percentage values were resolved to px lengths at cascade time for @viewport
widths and heights. This change keeps them as percentage values and recomputes
the actual viewport from the initial viewport and the percentage value when
the intial viewport changes (window resize). Likewise for viewport relative
units (vw, vh, vmin, vmax).

This fixes constrain-006 and constrain-007 in the w3c testsuite.

BUG=269132

Review URL: https://chromiumcodereview.appspot.com/22549002

git-svn-id: svn://svn.chromium.org/blink/trunk@157091 bbb929c8-8fbe-4397-9dbb-9b2b20218538