Disable flaky test CombinedShiftArrowPresses on Linux

The test has been flaky on Linux:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=browser_tests%20(with%20patch)&tests=PDFExtensionClipboardTest.CombinedShiftArrowPresses%2F

TBR=thestig@chromium.org
Bug: 1121446

Change-Id: Ibb0a5bb8f17e0d41a16f105d5ed6d4c3f1236dde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2558849Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830776}

chrome/browser/pdf/pdf_extension_test.cc

@@ -2284,7 +2284,14 @@ IN_PROC_BROWSER_TEST_P(PDFExtensionClipboardTest,
   SendCopyCommandAndCheckCopyPasteClipboard("HEL");
 }
 
-IN_PROC_BROWSER_TEST_P(PDFExtensionClipboardTest, CombinedShiftArrowPresses) {
+// Flaky on ChromeOS (https://crbug.com/1121446)
+#if defined(OS_LINUX)
+#define MAYBE_CombinedShiftArrowPresses DISABLED_CombinedShiftArrowPresses
+#else
+#define MAYBE_CombinedShiftArrowPresses CombinedShiftArrowPresses
+#endif
+IN_PROC_BROWSER_TEST_P(PDFExtensionClipboardTest,
+                       MAYBE_CombinedShiftArrowPresses) {
   LoadTestComboBoxPdfGetGuestContents();
 
   // Give the editable combo box focus.