Update webrequest.js

Fix three of the warnings generated by jshint (not sure where the redefined variable 'frames' is used in the code, so that's unchanged):

Four warnings
43	Use '!==' to compare with 'null'.
49	Redefinition of 'frames'.
59	Use '===' to compare with '0'.
148	Function declarations should not be placed in blocks. Use a function expression or move the statement to the top of the outer function.

It appears as if the variable 'frame' in the function isFrameWhitelisted() at line 130 is unused, but that could just be in this file and it could have significance generally.