This is a follow up on  issue 599201 .

The sequence for handling a notification click when Chrome has been killed (so a cold start) looks roughly like this:
04-04 11:38:36.529  3063  3063 D StatusBar: Clicked on content of 0|com.chrome.canary|-1|NotificationUIManager;https://tests.peter.sh/;1|10169
04-04 11:38:36.549 24367 24367 I NotificationService: Received a notification intent in the NotificationService's receiver.
04-04 11:38:36.817 24367 24367 I cr_LibraryLoader: Time to load native libraries: 226 ms (timestamps 5163-5389)
04-04 11:38:37.278 24367 24367 I NotificationUIManager: Dispatching notification event to native: 1
04-04 11:38:37.293 24367 24417 I chromium: [INFO:notification_event_dispatcher_impl.cc(165)] Lookup for ServiceWoker Registration: success: 1
04-04 11:38:37.310 24367 24417 I chromium: [INFO:notification_event_dispatcher_impl.cc(104)] Trying to dispatch notification for SW with status: 0
04-04 11:38:37.317   897  3506 I ActivityManager: Start proc 24429:com.chrome.canary:privileged_process0/u0a169 for service com.chrome.canary/org.chromium.content.app.PrivilegedProcessService0
04-04 11:38:37.409   897  3094 I ActivityManager: Start proc 24444:com.chrome.canary:sandboxed_process0/u0i77 for service com.chrome.canary/org.chromium.content.app.SandboxedProcessService0
04-04 11:38:37.613 24429 24457 I cr_LibraryLoader: Time to load native libraries: 166 ms (timestamps 6019-6185)
04-04 11:38:37.711 24444 24458 I cr_LibraryLoader: Time to load native libraries: 199 ms (timestamps 6084-6283)
04-04 11:38:37.951 24367 24417 I chromium: [INFO:notification_event_dispatcher_impl.cc(53)] The notification event has finished: 0
04-04 11:38:37.962   897  3094 I ActivityManager: START u0 {act=android.intent.action.VIEW dat=https://tests.peter.sh/... flg=0x10000000 pkg=com.chrome.canary cmp=com.chrome.canary/org.chromium.chrome.browser.document.ChromeLauncherActivity (has extras)} from uid 10169 on display 0
04-04 11:38:38.028   897  3464 I ActivityManager: START u0 {act=android.intent.action.VIEW dat=https://tests.peter.sh/... flg=0x14002000 pkg=com.chrome.canary cmp=com.chrome.canary/org.chromium.chrome.browser.ChromeTabbedActivity (has extras)} from uid 10169 on display 0
04-04 11:38:38.154   897  3094 I ActivityManager: Start proc 24471:com.chrome.canary:sandboxed_process1/u0i78 for service com.chrome.canary/org.chromium.content.app.SandboxedProcessService1
04-04 11:38:38.308   897   928 I ActivityManager: Displayed com.chrome.canary/org.chromium.chrome.browser.ChromeTabbedActivity: +263ms (total +325ms)
04-04 11:38:38.460 24471 24485 I cr_LibraryLoader: Time to load native libraries: 42 ms (timestamps 6990-7032)

In this case, it took 1.779 seconds to start showing a blank activity, the first time Chrome has shown a visible response to the user. The notification itself would have responded visually to a touch, but there's a long period of non-responsiveness in between.

This measurement was taken on a Nexus 5X, many devices are slower.

There are a few things that stand out in these logs:
* It looks like 2 renderers are started. First one for the service worker, then one for the site to be displayed. Could these things happen in the same process, especially as they are for the same origin?
* It looks like the gpu process is loaded quite early. Is this beneficial or does it slow down the time to first paint of an activity? I would not expect a service worker to need the gpu process.
* What is the distribution of timing data in production? I don't think we have instrumentation for the full sequence yet.
* It took over 600ms to dispatch the notification event. According to the ServiceWorker.NotificationClickEvent.Time histogram, in production 50% of users see this taking longer than 2.5 seconds.