console.context(name:string) method returns console instance, this console instance fully implements console interface (including fact that any method can be called without console as receiver).
Protocol.Runtime.consoleAPICalled notification contains additional context:string field:
- "anonymous#unique-id" for any method call on unnamed console context,
- "name#unique-id" for any method call on named console context.

console.count and console.timeEnd have context as a scope.
console.clear clear all messages regardless on what context instance it was called.

console calls is ~10% slower with this CL since we need to store and then fetch console_context_id and console_context_name from function object.
We recently (in April) made console calls twice faster so 10% doesn't sound critical and existing of console.log call in hot code is problem by itself.

R=pfeldman@chromium.org

Bug: chromium:728767
Change-Id: I5fc73216fb8b28bfe1e8c2c1b393ebfbe43cd02e
Reviewed-on: https://chromium-review.googlesource.com/522128Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45864}