Dashboard cleanup: Make builder a real dashboardSpecificParameter.
https://bugs.webkit.org/show_bug.cgi?id=108706

Reviewed by Dirk Pranke.

Add builder to g_defaultDashboardSpecificParameter maps for the
dashboards where it is used (flakiness, timeline, treemap).
Default it to null, and then update reads of it to check the
builder group's default if one is not set, rather than trying to
set and track this everywhere.
This allows us to remove tons of special-casing of builder parsing,
including where we used to add it to the query string even when the
user did not specify it.

* TestResultServer/static-dashboards/dashboard_base.js:
(parseParameters):
* TestResultServer/static-dashboards/flakiness_dashboard.js:
(generatePage):
(handleQueryParameterChange):
* TestResultServer/static-dashboards/loader.js:
(.):
* TestResultServer/static-dashboards/timeline_explorer.html:
* TestResultServer/static-dashboards/treemap.html:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141839 268f45cc-cd09-0410-ab3c-d52691b4dbfc