Only show notifications for supported events

Fixes the following:
1) We should only show notifications if we have a matching survey type.
2) We should only show notifications for extension install events (see Issue #71)

w/r/t extension install events, I'm matching against these:
https://code.google.com/p/chromium/codesearch#chromium/src/chrome/browser/extensions/extension_install_prompt.cc&l=194

I'm considering ExtensionInstallPrompt::INSTALL_PROMPT, ExtensionInstallPrompt::INLINE_INSTALL_PROMPT, and ExtensionInstallPrompt::BUNDLE_INSTALL_PROMPT as install events and ignoring all the rest. I've never seen the bundle install prompt but presumably it is not too different from the others.
The strings are built by the code that implements the experienceSamplingPrivate API. Unfortunately they aren't all defined in the same place --- rather the strings are defined at the call sites (ex in safe_browsing_blocking_page.cc). I'll add a comment to this effect.