WebPreferencesStore: use DEFINE_STATIC_LOCAL for overrides HashMap

WebPreferencesStore: use DEFINE_STATIC_LOCAL for overrides HashMap
https://bugs.webkit.org/show_bug.cgi?id=77109

Reviewed by Andreas Kling.

Fix build for Clang.

* Shared/WebPreferencesStore.cpp:
(WebKit):
(WebKit::boolTestRunnerOverridesMap):
(WebKit::WebPreferencesStore::overrideBoolValueForKey):
(WebKit::WebPreferencesStore::removeTestRunnerOverrides):
(WebKit::WebPreferencesStore::getBoolValueForKey):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@106008 268f45cc-cd09-0410-ab3c-d52691b4dbfc