[GTK] Use RunLoop::Timer in PluginInfoCache

[GTK] Use RunLoop::Timer in PluginInfoCache
https://bugs.webkit.org/show_bug.cgi?id=150757

Reviewed by Darin Adler.

Use RunLoop::Timer instead of GMainLoopSource to save the cache
contents in an idle. Also remove the mutex, since RunLoop::Timer
is thread safe and saveToFile() is private and only called by the
main thread loop. Also removed the code to save the file in the
destructor since PluginInfoCache is a singleton and never
destroyed. It's very unlikely that the program finished before the
cache is saved, but even if that happens it would be harmless, the
cache will be saved at some point the next time.

* UIProcess/Plugins/gtk/PluginInfoCache.cpp:
(WebKit::PluginInfoCache::PluginInfoCache):
(WebKit::PluginInfoCache::updatePluginInfo):
(WebKit::PluginInfoCache::~PluginInfoCache):
(WebKit::PluginInfoCache::saveToFile):
* UIProcess/Plugins/gtk/PluginInfoCache.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@191855 268f45cc-cd09-0410-ab3c-d52691b4dbfc