Revert 4 patches that deal with Extension ScriptContexts that have been submitted very recently.

      There are high crash rates caused by invalid ScriptContexts and it's unclear
which of these patches is causing them - potentially all of them. Reverting all
to be safe, and because they stomped on each other and can't be reverted
separately.

Revert &quotMove Extension ScriptContext creation into ScriptContextSet.&quot

This reverts commit 745b66237eca67b22225941eb475ba8a995e1287.

Revert &quot[Extensions] extension sendMessage respect to ActiveDOMObject policy&quot

This reverts commit 441ece58d63d2bf6e1f82e4055c71ef2f2ef63f4.

Revert &quotAdd back the call to extensions::ScriptContext::DispatchOnUnloadEvent.&quot

This reverts commit 0eda971a264a38756cc07416b1d1943b7e324aca.

Revert &quot[Extensions] Fix crash in ScriptContextSet&quot

This reverts commit a08a3109cc17941e39eb54b4d4223cfc14de83ae.

BUG=471599
TBR=rdevlin.cronin@chromium.org

Review URL: https://codereview.chromium.org/1064473003

Cr-Commit-Position: refs/heads/master@{#323869}