Add message validations

[lmuntaner]

Motivation

To validate the window post messages to avoid parsing messages that the code doesn't know what to do with them.

Changes

• Add validations in the handle function.
• Add comments to explain where the validation comes from.

Tests

• Added a test with "malicious" window messages from another origin and check that those don't affect the regular flow.

[frederikrothenberger]

LGTM, one minor comment.