Linux web_tool_tests is 1.12% flaky

[flutterflakybot]

The post-submit test builder Linux web_tool_tests had a flaky ratio 1.12% for the past 15 days, which is above our 1.00% threshold.

One recent flaky example for a same commit: https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_tool_tests/2827
Commit: flutter@d3c17ac
Failed build:
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_tool_tests/2827
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_tool_tests/2810

Succeeded build (3 most recent):
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_tool_tests/2907
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_tool_tests/2906
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_tool_tests/2905

Please follow https://github.com/flutter/flutter/wiki/Reducing-Test-Flakiness#fixing-flaky-tests to fix the flakiness and enable the test back after validating the fix (internal dashboard to validate: go/flutter_test_flakiness).