[NaCl SDK] Fix URLs in README.

No trybots because it is a documentation-only change.

BUG=137810
R=sbc@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/568153004

Cr-Commit-Position: refs/heads/master@{#294670}

native_client_sdk/src/README

@@ -10,7 +10,7 @@ Build Date: ${DATE}
 
 Please refer to the online documentation here:
 
-  https://developers.google.com/native-client
+  https://developer.chrome.com/native-client
 
 Contributing
 ------------
@@ -25,4 +25,4 @@ Known Issues
 
 Please refer to the online documentation here:
 
-  http://code.google.com/chrome/nativeclient/docs/releasenotes.html
+  https://developer.chrome.com/native-client/sdk/release-notes