robust_access_vertex shader add float tolerance

[shrekshao]

Issue: https://crbug.com/365974434

Bump up floating point number comparison with some tolerance. (As GLES highp float are subset of IEEE 754 32bit float)

This fix decrease local failing tests on Android pixel 6 chrome from 128 -> 93 74. I think newly passes ones are mostly control case flakes.

Still need to look into actual ooo failures.

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Requirements for PR author:

• All missing test coverage is tracked with "TODO" or .unimplemented().
• New helpers are /** documented */ and new helper files are found in helper_index.txt.
• Test behaves as expected in a WebGPU implementation. (If not passing, explain above.) (Still Failing compat android pixel 6 but fewer)
• Test have be tested with compatibility mode validation enabled and behave as expected. (If not passing, explain above.)

Requirements for reviewer sign-off:

• Tests are properly located in the test tree.
• Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Helpers and types promote readability and maintainability.

When landing this PR, be sure to make any necessary issue status updates.

[greggman]

lgtm with a couple of questions