REGRESSION (r171239): Much more time spent taking snapshots during the PLT

REGRESSION (r171239): Much more time spent taking snapshots during the PLT
https://bugs.webkit.org/show_bug.cgi?id=135177
<rdar://problem/17764847>

Reviewed by Dan Bernstein.

* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::willChangeCurrentHistoryItemForMainFrame):
(WebKit::WebPageProxy::willChangeCurrentHistoryItem): Deleted.
* UIProcess/WebPageProxy.h:
* UIProcess/WebPageProxy.messages.in:
* WebProcess/WebCoreSupport/WebFrameLoaderClient.cpp:
(WebKit::WebFrameLoaderClient::willChangeCurrentHistoryItem):
* WebProcess/WebPage/WebPage.cpp:
(WebKit::WebPage::willChangeCurrentHistoryItemForMainFrame):
(WebKit::WebPage::willChangeCurrentHistoryItem): Deleted.
* WebProcess/WebPage/WebPage.h:
Rename WillChangeCurrentHistoryItem to WillChangeCurrentHistoryItemForMainFrame.
Only send it when the current history item for the main frame changes.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@171471 268f45cc-cd09-0410-ab3c-d52691b4dbfc