Use unsigned int for data comparison inside ASSERT_GE()

Buildbot reports warning message "C4018: '>=' : signed/unsigned
mismatch" if the arguments of function ASSERT_GE() have different types.
Since runtimes.size() returns an unsigned int value, this CL changes the
other value to the same type for comparison.
Merge pull request #702 from shaochangbin/browsertest

Use unsigned int for data comparison inside ASSERT_GE()