Remove static initialization of user_agent_override_.

Convert to member variable.

Change-Id: I0985910a2730b3668796d706ab52cb12a7a8ba11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1659630
Commit-Queue: John Wu <jzw@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669921}

components/sessions/ios/ios_live_tab.h

@@ -52,7 +52,7 @@ class SESSIONS_EXPORT IOSLiveTab : public LiveTab,
   web::WebState* web_state_;
 
   // Needed to return an empty string in GetUserAgentOverride().
-  static std::string user_agent_override_;
+  std::string user_agent_override_;
 
   DISALLOW_COPY_AND_ASSIGN(IOSLiveTab);
 };

components/sessions/ios/ios_live_tab.mm

@@ -12,8 +12,6 @@ const char kIOSLiveTabWebStateUserDataKey[] = "ios_live_tab";
 
 namespace sessions {
 
-std::string IOSLiveTab::user_agent_override_;
-
 // static
 IOSLiveTab* IOSLiveTab::GetForWebState(web::WebState* web_state) {
   if (!web_state->GetUserData(kIOSLiveTabWebStateUserDataKey)) {