disables PDFExtensionClipboardTest.IndividualShiftLeftArrowPresses

It's flaky.

BUG=897801
TEST=test only change
TBR=thestig@chromium.org

Change-Id: Ieba62e4047a3fc240ac6b09b6e526ff87a0e29d3
Reviewed-on: https://chromium-review.googlesource.com/c/1294197Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601649}

chrome/browser/pdf/pdf_extension_test.cc

@@ -1620,8 +1620,9 @@ IN_PROC_BROWSER_TEST_F(PDFExtensionClipboardTest,
   SendCopyCommandAndCheckCopyPasteClipboard("HEL");
 }
 
+// TODO: test is flaky. https://crbug.com/897801
 IN_PROC_BROWSER_TEST_F(PDFExtensionClipboardTest,
-                       IndividualShiftLeftArrowPresses) {
+                       DISABLED_IndividualShiftLeftArrowPresses) {
   LoadTestComboBoxPdfGetGuestContents();
 
   // Give the editable combo box focus.