Fix timing-sensitive WebView test and improve framework.

Fix WebViewTest#testJavascriptInterfaceCustomPropertiesClearedOnReload
by using reloadAndWaitForCompletion() instead of reload(), so that the
reload actually completes before we check the results - the timing of
this changed slightly and the test became unreliable as a result.

Also, improve EvaluateJsResultPollingCheck - instead of polling until
the *expected* result is received, poll until *any* result is received
and then check if the result is the same as the expected result. This
means that in cases where the result is incorrect, it's reported by
assertEquals as a failure with the expected and actual result shown in
the test output, rather than appearing as a misleading timeout.

Bug: 71710844
Test: run CtsWebkitTestCases
Change-Id: I914089ba8cbbd094ad72f6dfed5ebf2176f9daf6
(cherry picked from commit f356a632)