[wk2] Scroll pinning state is wrong until the first scroll event

https://bugs.webkit.org/show_bug.cgi?id=127437

Reviewed by Simon Fraser.

* WebProcess/WebCoreSupport/WebFrameLoaderClient.cpp:
(WebKit::WebFrameLoaderClient::didChangeScrollOffset):
* WebProcess/WebPage/WebPage.cpp:
(WebKit::WebPage::updateMainFrameScrollOffsetPinning):
(WebKit::WebPage::didCommitLoad):
* WebProcess/WebPage/WebPage.h:
Rename WebPage::didChangeScrollOffsetForMainFrame to
updateMainFrameScrollOffsetPinning, which better describes what it
actually does. Call it when a load is committed, so that we have valid
pinning state before the first scroll event comes in.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@162548 268f45cc-cd09-0410-ab3c-d52691b4dbfc