[WebApk] Add initial support for WebShareTarget.

Adds the scaffolding to WebApk's main intent handling for
WebShareTarget.  Introduces a new launch type to track these metrics.
Primarily supports a {text} template variable as is typical for 
Android share events. Also supports {title} stemming from Intent.EXTRA_SUBJECT
for compatibility with WebShare

{url} as mentioned in the spec and any other {...} will be replaced by empty text.

Note: this a no-op as it is written since the conditional fields are
defaulted to off and unsupported on the server.

BUG=761007

Change-Id: I4f589034488771187341d4817708753ff74cc1b8
Reviewed-on: https://chromium-review.googlesource.com/734944
Commit-Queue: Peter Kotwicz <[email protected]>
Reviewed-by: Yaron Friedman <[email protected]>
Cr-Commit-Position: refs/heads/master@{#514070}
20 files changed