Regression fix: Remove spurious "CrOS" from CrOS UA string

This fixes a regression from [1], which accidentally modified CrOS'
User-Agent string.

Bug: 1061132
Change-Id: Ib42fe7eb840ceab38dba9a29d498b5c9ed041fa0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2102554Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Yoav Weiss <yoavweiss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750520}

content/common/user_agent.cc

@@ -116,7 +116,6 @@ std::string GetOSVersion(bool include_android_build_number) {
                       "%d_%d_%d", os_major_version, os_minor_version,
                       os_bugfix_version
 #elif defined(OS_CHROMEOS)
-                      "CrOS "
                       "%d.%d.%d",
                       os_major_version, os_minor_version, os_bugfix_version
 #elif defined(OS_ANDROID)