Bug 1637603 - Re-enable syncIPC test on Linux, r=emalysz.

Differential Revision: https://phabricator.services.mozilla.com/D75104

browser/base/content/test/performance/browser_startup_syncIPC.js

@@ -248,7 +248,7 @@ const startupPhases = {
     },
     {
       name: "PCompositorBridge::Msg_FlushRendering",
-      condition: MAC,
+      condition: MAC || LINUX,
       ignoreIfUnused: true,
       maxCount: 1,
     },

browser/base/content/test/performance/io/browser.ini

@@ -22,5 +22,4 @@ environment =
 [../browser_startup_mainthreadio.js]
 [../browser_startup_content_mainthreadio.js]
 [../browser_startup_syncIPC.js]
-skip-if = ((os == 'win') && asan) || (os == 'linux' && bits == 64 && os_version == '18.04') # Bug 1574535, Bug 1615516
-
+skip-if = ((os == 'win') && asan) # Bug 1629824