Implement Get Element Attribute WebDriver command
This intentionally doesn't implement the special handling for boolean
attributes yet, since that requires some kind of exhaustive list of all
such attributes
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Supports sending keys to an element. The specification here is still
rather unfinished so the error handling and so on in this code will
need iteration as it becomes clearer what the expected behaviour is.
This allows both boolean and string-type preferences. It
also implements a system where prefs that are read from a
configuration file can be reset back to their initial value,
which is useful in a number of cases e.g. when running tests
to ensure that each test starts with the same values for
the prefs.
Add get/set pref extensions to webdriver server.
This can be used by the test framework to ensure that the correct prefs are
set for a test without restarting the browser
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* Wait for the correct pipeline to return a LoadComplete message
before signalling a load is complete, and ensure that the root
pipeline is the one corresponding to the active document of the top
level browsing context, even if this has not yet painted.
* Ensure that TakeScreenshot operates on the correct pipeline
* Reset the screenshot ready flag whenever we decide that we are ready
to take a screenshot.
This relies on a global webdriverCallback function, which is visible to content.
Obviously that's not a long term solution for a number of reasons, but it allows
us to experiment for now
This is incomplete in several ways:
* It assumes that there's only one constellation (i.e. top level browsing context), ever.
* The session support is very basic indeed (no capabilities)
* Passing channels over channels may not sit well with IPC
* The error handling is mostly missing
