Rename MojoLayoutTestHelper to MojoWebTestHelper

mojo_layouttest_test.mojom is renamed to
mojo_web_test_helper_test.mojom. It keeps _test suffix indicating it
affects only tests.

Bug: 843412
Change-Id: Iddfdfcbfd87228fb9ee8abd2777f4838c4b66801
Reviewed-on: https://chromium-review.googlesource.com/c/1362717
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614302}
16 files changed