Need a WebKit2 API for setting media volume https://bugs.webkit.org/show_bug.cgi...

Need a WebKit2 API for setting media volume https://bugs.webkit.org/show_bug.cgi?id=76560

Reviewed by Dan Bernstein.

* Shared/WebPageCreationParameters.cpp:
(WebKit::WebPageCreationParameters::encode): Encode the mediaVolume parameter.
(WebKit::WebPageCreationParameters::decode): Decode the mediaVolume parameter.
* Shared/WebPageCreationParameters.h:
* UIProcess/API/C/WKPage.cpp:
(WKPageSetMediaVolume): Call WebPageProxy::setMediaVolume().
* UIProcess/API/C/WKPagePrivate.h:
* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::WebPageProxy): Initialize new data member m_mediaVolume.
(WebKit::WebPageProxy::setMediaVolume): Bail if the volume hasn't changed. Update m_mediaVolume
and bail if the page is no longer valid.  Otherwise, send a WebPage::SetMediaVolume message to
the web process.
(WebKit::WebPageProxy::creationParameters): Add media volume to the creation parameters.
* UIProcess/WebPageProxy.h:
* WebProcess/WebPage/WebPage.cpp:
(WebKit::WebPage::WebPage): Initialize media volume from the WebPageCreationParameters.
(WebKit::WebPage::setMediaVolume): Call Page::setMediaVolume().
* WebProcess/WebPage/WebPage.h:
* WebProcess/WebPage/WebPage.messages.in: Add the SetMediaVolume message.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105439 268f45cc-cd09-0410-ab3c-d52691b4dbfc