Use flutter_tools to start WebDriver browser.

[angjieli]

Description

Previously, when running Flutter web driver test, test code is responsible for starting driver browser which is anti-flutter patter (it DOES follows webdriver test pattern). This PR is going to put driver initialization logic into flutter_tools to be consistent.

Tests

I added the following tests:

Added test cased to 'packages/flutter_tools/test/general.shard/web/devices_test.dart' to verify ChromeDevice and WebServerDevice would return true for isWebDevice.

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[jonahwilliams]

Instead of adding isWebDevice you could check whether the target platform is web_javascript

[angjieli]

Fixed.

[jonahwilliams]

This seems like a bit of a roundabout way to specify the web client. Could supportTimelineAction be inferred here?

[angjieli]

Fixed by wrapping it into FlutterWebConnection.

[jonahwilliams]

You could expose a "supportsTImeline" getter on the driver, then you could wrap the TimelineSummary in that and merge this back with the other test script

[angjieli]

Discussed with Jonah offline. Going to keep this as it is for now so that we can verify flutter driver works with non-chrome browsers (testing on non-chrome browsers will throw exceptions when calling timeline-related actions).

[jonahwilliams]

LGTM