IDL compiler: sync Python to CL 26985002
IDL compiler: sync Python to CL 26985002

activeScriptExecutionContext => activeExecutionContext

Rename ScriptExecutionContext to ExecutionContext.
https://codereview.chromium.org/26985002

NOTRY=true

Review URL: https://codereview.chromium.org/26591005

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@159628 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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