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 recompute
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.

This CL also disallows 'initial' and 'inherit' as descriptor values and returns 'auto' for unknown non-primitive values to avoid crashers and improve on the original CL for the same bug: https://codereview.chromium.org/22549002/

BUG=269132

Review URL: https://chromiumcodereview.appspot.com/23604037

git-svn-id: svn://svn.chromium.org/blink/trunk@157225 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Revert r155355 as it caused CJK glyphs to render as boxes for users with Droid Sans installed.

I've filed http://crbug.com/285303 about fixing the underlying issue
that this change made more obvious.

Unfortunately this change will cause a large perf regression for CJK text.
It's just reverting a large perf gain we got with the previous change.

This reverts:
https://codereview.chromium.org/21497002

BUG=272006
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/23622018

git-svn-id: svn://svn.chromium.org/blink/trunk@157226 bbb929c8-8fbe-4397-9dbb-9b2b20218538
New UMA stats for stacking container opt-in measurements.

Along with https://codereview.chromium.org/23874003/, this
adds Renderer.CompositedScrolling UMA histogram.

This records the total number of RenderLayers that are scrollable
areas, need to become stacking containers, and will use composited
scrolling.

Each of the buckets should be recorded at most once per
RenderLayer. The ratio between these stats should give
us an idea of how effective stacking containers are at
allowing scrollable divs to become composited.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23455031

git-svn-id: svn://svn.chromium.org/blink/trunk@157227 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Auto-rebaseline for r157175

http://src.chromium.org/viewvc/blink?view=revision&revision=157175

TBR=ojan@chromium.org

Review URL: https://codereview.chromium.org/23431016

git-svn-id: svn://svn.chromium.org/blink/trunk@157228 bbb929c8-8fbe-4397-9dbb-9b2b20218538