Fix minidump size limit used for MinidumpSizeLimit unittest

Fix minidump size limit used for MinidumpSizeLimit unittest

If the stack sizes for threads in the MinidumpSizeLimit test are too big,
then subtracting 64KB from the normal minidump file size is not enough to
trigger the size-limiting logic.  Instead of basing the arbitrary limit off
of the normal file size, make it relative to the 8KB stack size the logic
assumes.

BUG=google-breakpad:510
TEST=Ran unittests
Committed: https://code.google.com/p/google-breakpad/source/detail?r=1090

[Ivan Penkov]

This looks good.  I verified that the test now passes in google3:

//third_party/breakpad:google/test/minidump_writer_unittest              PASSED
in 1.6s

Thanks,
-Ivan