Reland "Ice Server API integration"

This is a reland of b23ba4d3fb

Reason for revert: Did not roll out internal DEPS
(https://chrome-internal-review.googlesource.com/c/chrome/src-internal/+/2450652)

TBR=msramek@chromium.org

Original change's description:
> Ice Server API integration
>
> Integrates network traversal API to fetch stun and turn servers for
> webrtc p2p connection. Handles API failure by returning a list of
> public Google stun servers as a fallback.
>
> Bug: 1031156
> Change-Id: I60b5d6ccdb749ce9375bcb3333dc2cbdab62aa18
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1994295
> Commit-Queue: Himanshu Jaju <himanshujaju@chromium.org>
> Reviewed-by: Martin Šrámek <msramek@chromium.org>
> Reviewed-by: David Roger <droger@chromium.org>
> Reviewed-by: Richard Knoll <knollr@chromium.org>
> Reviewed-by: Alex Gough <ajgo@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#733941}

Bug: 1031156
Change-Id: Ib1f409842a3277b5e1f47d6511ede46361aa5fc8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2013130
Commit-Queue: Himanshu Jaju <himanshujaju@chromium.org>
Reviewed-by: Richard Knoll <knollr@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734399}

google_apis/google_api_keys.h

@@ -76,6 +76,9 @@ std::string GetNonStableAPIKey();
 // Retrieves the Chrome Remote Desktop API key.
 std::string GetRemotingAPIKey();
 
+// Retrieves the Sharing API Key.
+std::string GetSharingAPIKey();
+
 #if defined(OS_IOS)
 // Sets the API key. This should be called as early as possible before this
 // API key is even accessed.