Findit identified the culprit r580541 with confidence 70.0% in the config "tryserver.chromium.win / win10_chromium_x64_rel_ng"
based on the flakiness trend:

https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVy7wELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCK4AWNocm9taXVtLndpbi9XaW4xMCBUZXN0cyB4NjQvMjYxOTkvY2NfdW5pdHRlc3RzIG9uIFdpbmRvd3MtMTAtMTUwNjMvVEdGNVpYSlVjbVZsU0c5emRFSnNaVzVrYVc1blVHbDRaV3hVWlhOMExrSnNaVzVrYVc1blYybDBhRkpsYm1SbGNsQmhjM05UYUdGa1pYSnpWMmwwYUUxaGMydEJRVjlIVEY5VVpYaDBkWEpsVW1WamRBPT0MCxITTWFzdGVyRmxha2VBbmFseXNpcxgBDA

If the culprit above is wrong, please file a bug using this link and hit submit:
https://bugs.chromium.org/p/chromium/issues/entry?status=Unconfirmed&labels=Pri-1,Test-Findit-Wrong&components=Tools%3ETest%3EFindit%3EFlakiness&summary=%5BFindit%5D%20Flake%20Analyzer%20-%20Wrong%20result%20for%20LayerTreeHostBlendingPixelTest.BlendingWithRenderPassShadersWithMaskAA_GL_TextureRect&comment=Link%20to%20Analysis%3A%20https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVy7wELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCK4AWNocm9taXVtLndpbi9XaW4xMCBUZXN0cyB4NjQvMjYxOTkvY2NfdW5pdHRlc3RzIG9uIFdpbmRvd3MtMTAtMTUwNjMvVEdGNVpYSlVjbVZsU0c5emRFSnNaVzVrYVc1blVHbDRaV3hVWlhOMExrSnNaVzVrYVc1blYybDBhRkpsYm1SbGNsQmhjM05UYUdGa1pYSnpWMmwwYUUxaGMydEJRVjlIVEY5VVpYaDBkWEpsVW1WamRBPT0MCxITTWFzdGVyRmxha2VBbmFseXNpcxgBDA

Automatically posted by the findit-for-me app (https://goo.gl/Ot9f7N).

I had cc'd the wrong person.

Is it correct that the flakiness increased only for the config "tryserver.chromium.win / win10_chromium_x64_rel_ng"?

Because the found culprit is an Android-only change (it doesn't even change any native code). This would make it very unlikely that it's the true cause.