[ES6 modules] WPT: mark error test cases as flaky

The web platform tests testing module script errors are not deterministic
until we have the new algorithm being proposed at HTML PRs below:
- whatwg/html#2971
- whatwg/html#2991

This CL marks the test as flaky until we land the new algorithm.

TBR=hiroshige@chromium.org

Bug: 767093, 763597
Change-Id: I9ef17f2ae6dc51acbef4dbab1f7f7c7438d17eb7
Reviewed-on: https://chromium-review.googlesource.com/676403
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#503304}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3053,6 +3053,9 @@ crbug.com/736056 [ Mac Win7 ] external/wpt/encoding/legacy-mb-korean [ Timeout P
 crbug.com/736056 [ Mac ] external/wpt/encoding/legacy-mb-tchinese [ Timeout Pass ]
 crbug.com/736056 external/wpt/encoding/legacy-mb-korean/euc-kr/euckr-encode-form-errors-hangul.html [ Timeout Pass ]
 
+# Known issue: module script errors are not derteministic
+crbug.com/763597 external/wpt/html/semantics/scripting-1/the-script-element/module/instantiation-error-5.html [ Failure Pass ]
+
 # module script lacks XHTML support
 crbug.com/717643 external/wpt/html/semantics/scripting-1/the-script-element/module/module-in-xhtml.xhtml [ Failure ]