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::appendToWebDisplayItemList(WebDisplayItemList* list)
{
    for (const DisplayItem& item : m_currentDisplayItems)
        item.appendToWebDisplayItemList(list);
}

void DisplayItemList::appendToWebDisplayItemList(WebDisplayItemList* list)
{
    for (const DisplayItem& item : m_currentDisplayItems)
        item.appendToWebDisplayItemList(list);
}
