Fix mirror-script and debug-script tests when using external natives.

R=yangguo
BUG=

Review URL: https://codereview.chromium.org/792733003

Cr-Commit-Position: refs/heads/master@{#25752}
Make d8 default to standard location for external snapshots.

This makes tests runnable with the external snapshot, and should be the
last step before enabling external snapshot on >=1 bots.

R=yangguo
BUG=

Review URL: https://codereview.chromium.org/780333004

Cr-Commit-Position: refs/heads/master@{#25753}
Avoid number range holes in bitset types.

BUG=

Review URL: https://codereview.chromium.org/759013003

Cr-Commit-Position: refs/heads/master@{#25754}
Skip slow webkit/array-iterate-backwards in arm64.debug/gc-stress mode.

BUG=
TBR=hpayer@chromium.org

Review URL: https://codereview.chromium.org/795633002

Cr-Commit-Position: refs/heads/master@{#25755}