LaunchParams needs share data

[alancutter]

If a web app is a share target and uses "launch_handler": { "client_mode": "focus-existing" } the share payload is dropped. LaunchParams should include a share_data member for this situation.

Bug report: https://bugs.chromium.org/p/chromium/issues/detail?id=1213776

This proposal relates to: w3c/web-share-target#84

[alancutter]

Looks like the data contents of https://w3c.github.io/web-share-target/level-2/#sharetargetparams-and-its-members needs to be turned into a JS object.

[alancutter]

After discussions with @ericwilligers it sounds like we can package up the share data as a FormData object.
Maybe the share target level 2 spec could be refactored to define how to construct the FormData and then call into existing procedures for producing an HTTP request out of it.