Fix some dead links in docs

1. Remove the links of deleted files
  - docs/common_build_tasks.md
  - docs/shift_based_development.md
  deleted in https://chromium-review.googlesource.com/679054
  - docs/subtle_threading_bugs.md
  deleted in https://chromium-review.googlesource.com/897782

2. Remove Old Chrome OS build instructions and
   add Chrome OS build instructions in Checking Out and Building
  - docs/old_chromeos_build_instructions.md
  renamed in https://chromium-review.googlesource.com/861043

3. Remove the link of an unidentified file
  - docs/testing/test_browser_ui.md
  can't find the history of the file.

Bug: N/A
Change-Id: I4c6d3e6fbe442ad79905c94b94627ad8baf251dd
Reviewed-on: https://chromium-review.googlesource.com/1029396
Commit-Queue: Hwanseung Lee <hs1217.lee@samsung.com>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554686}
diff --git a/docs/get_the_code.md b/docs/get_the_code.md
index 0d637d4..e4bb6e35 100644
--- a/docs/get_the_code.md
+++ b/docs/get_the_code.md
@@ -10,9 +10,10 @@
 you might want to build:
 
 * [Android](android_build_instructions.md)
-* [Cast](old_cast_build_instructions.md)
-* [Chrome OS](old_chromeos_build_instructions.md)
+* [Android Cast](android_cast_build_instructions.md)
+* [Chrome OS](chromeos_build_instructions.md)
 * [iOS](ios/build_instructions.md)
 * [Linux](linux_build_instructions.md)
+* [Linux Cast](linux_cast_build_instructions.md)
 * [Mac](mac_build_instructions.md)
 * [Windows](windows_build_instructions.md)