[inspector-protocol] exposed Runtime.ExecutionContextId

TBR=dgozman@chromium.org

Bug: none
Change-Id: Idc0020b44140b8c319622950aa309723c4e086eb
Reviewed-on: https://chromium-review.googlesource.com/544964Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46148}

src/inspector/inspector_protocol_config.json

@@ -12,7 +12,7 @@
             {
                 "domain": "Runtime",
                 "async": ["evaluate", "awaitPromise", "callFunctionOn", "runScript"],
-                "exported": ["StackTrace", "RemoteObject"]
+                "exported": ["StackTrace", "RemoteObject", "ExecutionContextId"]
             },
             {
                 "domain": "Debugger",