Turn on flag for Linux, too.

chrome/browser/about_flags.cc

Index: chrome/browser/about_flags.cc
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 54f5e55c6b1a52078629ee87f95e28e05e4e22d9..ba3f3b4d48393f82f928ca0d4eb105dabb539cbe 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -130,13 +130,13 @@ const Experiment kExperiments[] = {
     IDS_FLAGS_CLOUD_PRINT_PROXY_NAME,
     IDS_FLAGS_CLOUD_PRINT_PROXY_DESCRIPTION,
 #if defined(GOOGLE_CHROME_BUILD)
-    // For a Chrome build, we know we have a PDF plug-in, and so we'll
-    // enable by platform as we get things working.
-    kOsMac,
+    // For a Chrome build, we know we have a PDF plug-in on Windows, so it's
+    // fully enabled. Mac and Linux still need some final polish.
+    kOsMac | kOsLinux,
 #else
     // Otherwise, where we know it could be working if a viable PDF
     // plug-in could be supplied, we'll keep the lab enabled.
-    kOsWin | kOsMac,
+    kOsWin | kOsMac | kOsLinux,
 #endif
     SINGLE_VALUE_TYPE(switches::kEnableCloudPrintProxy)
   },