[Isolate] Update clients to hit /_ah/api instead of /api

The webapp2 adapter is now serving at /_ah/api, the old Endpoints v1 path.

https://chromium.googlesource.com/infra/infra revision:
d8ec646a9503426a5018cf5bc251abe4a842357f

https://chromium.googlesource.com/infra/luci/luci-go revision:
928030a644473add1ca1bd044d13f48fee569c1a

linux64/isolate.sha1:
https://ci.chromium.org/buildbot/chromium.infra/infra-continuous-trusty-64/12546

mac64/isolate.sha1:
https://ci.chromium.org/buildbot/chromium.infra/infra-continuous-mac-10.13-64/4878

windows64/isolate.exe.sha1:
https://ci.chromium.org/buildbot/chromium.infra/infra-continuous-win-64/12934

Bug: 805543
Change-Id: I4b343a8297425ae855fb598f6249fb3d481010a3
Reviewed-on: https://chromium-review.googlesource.com/1083844
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: smut <smut@google.com>
Cr-Original-Commit-Position: refs/heads/master@{#563907}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 289dfc08a4bfecbdca3e743b716d7d1bd6f251d6
3 files changed