There seems to be some sort of race condition, as this test flakes for me with --enable-pixel-output-in-tests or with --enable-gpu-rasterization.

When it flakes, it seems like DragController::canProcessDrag(DragData* dragData) is returning false because result.innerNode()->hasEditableStyle() is false (on drag enter in the embedded renderer), though I don't understand enough about this code to know why that would be the case.

Test output:

$ out/Release/browser_tests.exe --gtest_filter=WebUIWebViewBrowserTest.DragAndDropToInput --gtest_repeat=100 --enable-gpu-rasterization

[ RUN      ] WebUIWebViewBrowserTest.DragAndDropToInput
[15672:11424:0519/180003:INFO:CONSOLE(7538)] "Running test testDragAndDropToInput", source: webview_basic.js (7538)
[15672:11424:0519/180003:INFO:CONSOLE(11041)] "load stop of src = :about:blank", source: webview_basic.js (11041)
[15672:11424:0519/180003:INFO:CONSOLE(11052)] "[Guest]: start guest js", source: webview_basic.js (11052)
[15672:11424:0519/180003:INFO:CONSOLE(11052)] "[Guest]: finish guest js", source: webview_basic.js (11052)
[15672:11424:0519/180003:INFO:CONSOLE(11044)] "finish guest load", source: webview_basic.js (11044)
[15672:11424:0519/180003:INFO:CONSOLE(11052)] "[Guest]: guest gets message create-channel", source:
webview_basic.js (11052)
[15672:11424:0519/180003:INFO:CONSOLE(11052)] "[Guest]: guest posts message: connected", source: webview_basic.js (11052)
[15672:11424:0519/180003:INFO:CONSOLE(11029)] "get message: connected", source: webview_basic.js (11029)
[15672:11424:0519/180003:INFO:CONSOLE(1)] "step1: Drag Enter", source:  (1)

and it hangs there.