Remove tracking of pending sheets.

I was going to remove all this anyway since we don't need it in sky, all sheets
are local and there's no concept of a pending sheet now.

I also removed the dirty bit I added to StyleSheetCollection. The bit
is not correct and is preventing us from correctly processing sheets and
invalidating style. I'll add it back later when I understand how to add
the dirty bit correctly.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/846183002

engine/src/flutter/tests/lowlevel/home-expected.txt

@@ -0,0 +1,13 @@
+layer at (0,0) size 800x600
+  RenderView {#document} at (0,0) size 800x600
+layer at (0,0) size 800x137
+  RenderBlock {div} at (0,0) size 800x137
+    RenderBlock {div} at (32,32) size 736x38
+      RenderParagraph (anonymous) at (0,0) size 736x38
+        RenderText {#text} at (0,0) size 191x38
+          text run at (0,0) width 191: "about:blank"
+    RenderBlock {div} at (16,110) size 768x19 [color=#BCD8F5]
+      RenderParagraph (anonymous) at (0,0) size 768x19
+        RenderText {#text} at (0,0) size 152x19
+          text run at (0,0) width 152: "Welcome to Sky!"
+

engine/src/flutter/tests/lowlevel/home.sky

@@ -0,0 +1,14 @@
+<div>
+<style>
+ .h1 { font-size: 2em; margin: 1em; }
+ .p { margin: 0.5em 1em; color: #bcd8f5; font-weight: 900; }
+</style>
+<div class="h1">about:blank</div>
+<div class="p">Welcome to Sky!</div>
+<script>
+// Don't use dump-as-render-tree.sky, we want a test with no imports.
+window.addEventListener('load', function() {
+  internals.notifyTestComplete(internals.renderTreeAsText());
+});
+</script>
+</div>