DevTools: allow embedder handling async remote debugger commands.

content/browser/devtools/protocol_config.json

 {
     "use_snake_file_names": true,
     "use_title_case_methods": true,
 
     "protocol": {
         "package": "content/browser/devtools/protocol",
         "output": "protocol",
         "namespace": ["content", "protocol"],
         "export_macro": "CONTENT_EXPORT",
         "export_header": "content/common/content_export.h",
@@ skipping 28 matching lines @@
                 "domain": "Network",
                 "include": ["enable", "disable", "clearBrowserCache", "clearBrowserCookies", "getCookies", "getAllCookies", "deleteCookie", "setCookie", "setUserAgentOverride", "canEmulateNetworkConditions"],
                 "include_types": ["CookieSameSite", "Cookie", "Response", "Headers", "Request", "ResourceTiming", "SecurityDetails", "SignedCertificateTimestamp", "Initiator", "ResourcePriority", "RequestWillBeSentNotification", "ResponseReceivedNotification", "LoadingFinishedNotification", "LoadingFailedNotification", "RequestWillBeSentNotification"],
                 "include_events": ["requestWillBeSent", "responseReceived", "loadingFinished", "loadingFailed"],
                 "async": ["getCookies", "getAllCookies", "deleteCookie", "setCookie"]
             },
             {
                 "domain": "Page",
                 "include": ["enable", "disable", "reload", "navigate", "stopLoading", "getNavigationHistory", "navigateToHistoryEntry", "captureScreenshot",
                     "startScreencast", "stopScreencast", "screencastFrameAck", "handleJavaScriptDialog", "setColorPickerEnabled", "requestAppBanner",
-                    "setControlNavigations", "processNavigation"],
+                    "setControlNavigations", "processNavigation", "printToPDF"],
                 "include_events": ["interstitialShown", "interstitialHidden", "navigationRequested", "screencastVisibilityChanged", "screencastFrame", "colorPicked"],
-                "async": ["captureScreenshot"]
+                "async": ["captureScreenshot", "printToPDF"]
             },
             {
                 "domain": "Runtime",
                 "include_types": ["StackTrace", "CallFrame"],
                 "include_events": [],
                 "include": []
             },
             {
                 "domain": "Schema"
             },
@@ skipping 25 matching lines @@
     },
 
     "lib": {
         "package": "content/browser/devtools/protocol",
         "output": "protocol",
         "string_header": "content/browser/devtools/protocol_string.h",
         "export_macro": "CONTENT_EXPORT",
         "export_header": "content/common/content_export.h"
     }
 }