Time To Interactive Definition

As discussed in our last meeting, here
<https://docs.google.com/document/d/1l50v3ljDeUmj9qBsfdPc3wKe_1CY5KFYI1baye2NhYk/edit?ts=59dfaf65#>'s
a description of Chrome's current implementation of Time to Interactive.

Here
<https://docs.google.com/document/d/1y6NYcI2NyLMh88KVr2NaevdX2e-e03vL99Wn-gy_hVk/edit#>'s
some rough brainstorming on the next step for Chrome's implementation.

At TPAC, we're hoping to discuss:

   - The high level objective of TTI.
   - Chrome's current algorithm for computing TTI.
   - Enabling TTI measurement in the wild.

Tim