The callback parameter of WebBrowserComponent::evaluateJavascript has
a default value of nullptr. Updated the Linux implementation to handle
this case properly by checking if the callback queue is empty before
attempting to invoke the callback.
This commit reverts 8e6aeab799.
The WebBrowserComponent subprocess calls tryNextRead() in an infinite
loop. Prior to the reverted change this allowed it to handle the
transfer of larger files, which would span multiple calls to the
function. The transfer state would be remembered in the receivingLength
and pos class members.
The simplification in 8e6aeab799 mainly
comes from moving these class members into function locals, but this
means, that the transfer state is lost whenever the break statements are
hit. This would cause bad access during the transfer of larger files.
