Mark HStringCompareAndBranch as potentially causing GCs.

This also adds a %SetAllocationTimout runtime function which helps to
write regression tests that need to trigger a GC at a certain point in
program execution.

R=hpayer@chromium.org
BUG=chromium:274438
TEST=mjsunit/regress/regress-crbug-274438

Review URL: https://codereview.chromium.org/22933006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16205 ce2b1a6d-e550-0410-aec6-3dcde31c8c00