Rename some symbols in content/shell/common/web_test from "layout test" to "web test"

- content::LayoutTestContentClient
- Message class 'LayoutTeestHostMsg' and message names
- Header guard macro,
  CONTENT_SHELL_COMMON_LAYOUT_TEST_LAYOUT_TEST_MESSAGES_H_

This CL also renames some symbols around the above symbols.
This CL has no behavior changes.

TBR=inferno@chromium.org

Bug: 843412
Change-Id: I2657f2b85de138e5653341fb49055a7f6aa608be
Reviewed-on: https://chromium-review.googlesource.com/c/1360210
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613845}
20 files changed