[WebView] fix the race with generate/copy the InspectorJS* files

The WebView build may fail with the following error:

test -f generated/InspectorJSFrontendDispatchers.cpp || python <http://jfx.us.oracle.com/hudson/job/9-web-sandbox/./label=linux-amd64-90X/ws/rt/modules/web/src/main/native/Source/JavaScriptCore/inspector/scripts/CodeGeneratorInspector.py> generated/InspectorJS.json --output_h_dir generated --output_cpp_dir generated --output_js_dir generated --output_type JavaScript && cd generated && test -d inspector || mkdir -p inspector && cp -f InspectorJS*.h inspector
test -f generated/InspectorJSBackendDispatchers.cpp || python <http://jfx.us.oracle.com/hudson/job/9-web-sandbox/./label=linux-amd64-90X/ws/rt/modules/web/src/main/native/Source/JavaScriptCore/inspector/scripts/CodeGeneratorInspector.py> generated/InspectorJS.json --output_h_dir generated --output_cpp_dir generated --output_js_dir generated --output_type JavaScript && cd generated && test -d inspector || mkdir -p inspector && cp -f InspectorJS*.h inspector
cp: cannot create regular file ?inspector/InspectorJSBackendDispatchers.h?: File exists
make[1]: *** [generated/InspectorJSFrontendDispatchers.cpp] Error 1

This is caused by the running the generator/copy instructions in parallel which ends up with "cp -f" getting into the race.