Fix handling of Javascript functions from Rewards WebUI pages

@bridiver noticed a few problems in the Rewards WebUI pages:

1. We shouldn’t call CallJavascriptFunctionUnsafe. Instead, use WebUIMessageHandler::CallJavascriptFunction or FireWebUIListener, since those are safe alternatives.
2. We should call IsJavascriptAllowed instead of CanCallJavascript.
3. Ensure that we’re calling AllowJavascript correctly, per the docs in the header file. It’s sometimes not called when it should be and sometimes called when it shouldn’t be.

[SergeyZhukovsky]

that call

GetMojomFrameInRenderer()->JavaScriptExecuteRequest(javascript, wants_result,
                                                      std::move(callback));

isn’t going throw where javascript is the actual script to be executed. In what place couldn’t it find the function? that’s the crash place before any validations GetMojomFrameInRenderer()->. It would crash constantly and always the way you explain it. But in fact we cannot replicate that, we just see crashes from users.

[zenparsing]

Right, because CallJavascriptFunctionUnsafe can’t find the named JS function “brave_rewards.statement” in the Android version of the rewards page. We really need to get a native version of the Rewards page for Android! 😄

…although in this case the android page appears to provide that function, so there’s probably something more going on.