Speculatively launch Service Workers on mouse/touch events. [5/5]

1/5: Introduce NavigationHintSender.
     https://codereview.chromium.org/2043863003/
2/5: Pipe NavigationHints from NavigationHintSender to ChromeRenderMessageFilter
     https://codereview.chromium.org/2043083002/
3/5: Call StartServiceWorkerForNavigationHint() from ChromeRenderMessageFilter
     https://codereview.chromium.org/2052613003/
4/5: Measure the precision of the speculative launch of Service Workers for
NavigationHints
     https://codereview.chromium.org/2045153003/
5/5: Add flags to enable SupeculativeLaunchServiceWorker
     This CL.

This CL introduces a new feature named "SpeculativeLaunchServiceWorker".
When the feature is enabled, Service Workers are speculatively started on the
user's mousedown/touch events.
Demo:
https://drive.google.com/file/d/0B6skYAFVnosEMWdzRm4yMEtIMkk/view?usp=sharing

BUG=616502

Review-Url: https://codereview.chromium.org/2053573002
Cr-Commit-Position: refs/heads/master@{#410295}
11 files changed