[Sampling profiler] Avoid underflow in OS X rbp check

When rbp was less than the offset, the check was underflowing and
incorrectly passing. Add a check for rbp < offset to detect this
situation.

Bug: 831448
Change-Id: I4f780c1573782af3ca29b0f814e93ff2dca839f2
Reviewed-on: https://chromium-review.googlesource.com/1013213Reviewed-by: Leonard Grey <lgrey@chromium.org>
Commit-Queue: Mike Wittman <wittman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551113}

base/profiler/native_stack_sampler_mac.cc

@@ -328,8 +328,8 @@ bool WalkStackFromContext(
       unw_word_t rsp, rbp;
       unw_get_reg(&unwind_cursor, UNW_X86_64_RSP, &rsp);
       unw_get_reg(&unwind_cursor, UNW_X86_64_RBP, &rbp);
-      uint32_t offset = GetFrameOffset(proc_info.format);
-      if ((rbp - offset * 8) < rsp || rbp > stack_top) {
+      uint32_t offset = GetFrameOffset(proc_info.format) * sizeof(unw_word_t);
+      if (rbp < offset || (rbp - offset) < rsp || rbp > stack_top) {
         return false;
       }
     }