lacros: Temporarily revert back to Linux user-agent.

The transition to Lacros is intended to be transparent to websites. As
such, Lacros needs to set the ChromeOS user-agent. However, in some
contexts (e.g. chrome://settings) Lacros must behave like the Linux
browser. The problem is that webUI makes the assumption that user-agent
uniquely identifies platform, and that there is only one browser per
platform. Thus, setting the ChromeOS user-agent triggers ChromeOS
settings behavior, which uses ChromeOS-only private extension APIs,
which crashes the renderer.

The appropriate long-term fix to this is to correctly plumb additional
information into WebUI so that it can distinguish between Linux,
ChromeOS and Lacros browsers. This is currently not possible without
refactoring a substantial number of tests. This CL is a stopgap measure
to allow chrome://settings to be usable in Lacros.

Bug: 1096590, 1111985
Change-Id: Iabb8691e37ba6db4baba680e3d624248e8647321
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367473Reviewed-by: Darin Fisher <darin@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Auto-Submit: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800292}

content/common/user_agent.cc

@@ -123,8 +123,8 @@ std::string GetLowEntropyCpuArchitecture() {
 std::string GetOSVersion(IncludeAndroidBuildNumber include_android_build_number,
                          IncludeAndroidModel include_android_model) {
   std::string os_version;
-#if defined(OS_WIN) || defined(OS_MAC) || defined(OS_CHROMEOS) || \
-    BUILDFLAG(IS_LACROS)
+#if defined(OS_WIN) || defined(OS_MAC) || \
+    (defined(OS_CHROMEOS) && !BUILDFLAG(IS_LACROS))
   int32_t os_major_version = 0;
   int32_t os_minor_version = 0;
   int32_t os_bugfix_version = 0;
@@ -144,7 +144,7 @@ std::string GetOSVersion(IncludeAndroidBuildNumber include_android_build_number,
 #elif defined(OS_MAC)
                       "%d_%d_%d", os_major_version, os_minor_version,
                       os_bugfix_version
-#elif defined(OS_CHROMEOS) || BUILDFLAG(IS_LACROS)
+#elif defined(OS_CHROMEOS) && !BUILDFLAG(IS_LACROS)
                       "%d.%d.%d", os_major_version, os_minor_version,
                       os_bugfix_version
 #elif defined(OS_ANDROID)
@@ -185,7 +185,7 @@ std::string BuildOSCpuInfoFromOSVersionAndCpuType(const std::string& os_version,
   base::StringAppendF(&os_cpu,
 #if defined(OS_MAC)
                       "%s Mac OS X %s", cpu_type.c_str(), os_version.c_str()
-#elif defined(OS_CHROMEOS) || BUILDFLAG(IS_LACROS)
+#elif defined(OS_CHROMEOS) && !BUILDFLAG(IS_LACROS)
                       "CrOS "
                       "%s %s",
                       cpu_type.c_str(),  // e.g. i686