Create PaintChunk and begin writing code to build paint chunks.

BUG=537409

Review URL: https://codereview.chromium.org/1379883003

Cr-Commit-Position: refs/heads/master@{#352369}
 void DisplayItemList::beginScope()
{
    ASSERT_WITH_SECURITY_IMPLICATION(m_nextScope < UINT_MAX);
    m_scopeStack.append(m_nextScope++);
    beginSkippingCache();
}

 void DisplayItemList::beginScope()
{
    ASSERT_WITH_SECURITY_IMPLICATION(m_nextScope < UINT_MAX);
    m_scopeStack.append(m_nextScope++);
    beginSkippingCache();
}
