Use a dirty bit in StyleSheetCollection::updateActiveStyleSheets.

Use a dirty bit in StyleSheetCollection::updateActiveStyleSheets.

The loop in StyleEngine::updateActiveStyleSheets calls this
for every scope whenever any stylesheet is added or removed
from the document, ex. appending a new SkyElement with a
<style> in it. The method was crawling the scope and looking
for all sheets and then marking the element as needing a
SubtreeStyleRecalc, instead keep a dirty bit to avoid doing
a full document recalc all the time.

I made this slow previously when I removed all the dirty tree
scope tracking logic and made us go down the reconstruct path
in the resolver update all the time.

We now keep a dirty bit and only do this work when the sheets
for a scope have changed.

R=ojan@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/1a2736821a99a68d65a9508d938a4967fe49ef40

[esprehn, 2015-01-10 04:25:56]

esprehn@chromium.org changed reviewers:
+ ojan@chromium.org

[ojan, 2015-01-10 04:32:49]

lgtm

[esprehn, 2015-01-10 04:33:34]

Committed patchset #1 (id:1) manually as
1a2736821a99a68d65a9508d938a4967fe49ef40 (presubmit successful).