Currently we have four ImeTextSpan class/structs

1) ui::ImeTextSpan [1]
2) blink::ImeTextSpan [2]
3) blink::WebImeTextSpan [3]
4) mojom::ImeTextSpanDataView [4]

They are almost identical to each other, ideally we could use one mojom class to replace all the others, so we could avoid to converting them back and forth. The mojom generated class could be used in Java side directly. It also probably improves performance slightly.

One difficulty I am aware of is that blink::WebImeTextSpan is also used in IPC code of browser plugin [5], I am not familiar with that code path, but we probably need to mojolize that code first. 

[1] https://cs.chromium.org/chromium/src/ui/base/ime/ime_text_span.h
[2] https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/editing/ime/ime_text_span.h
[3] https://cs.chromium.org/chromium/src/third_party/blink/public/web/web_ime_text_span.h
[4] https://cs.chromium.org/chromium/src/services/ws/public/mojom/ime/ime.mojom?rcl=db55c1a8af9f153fc8c297fcc5735dabfecba4ee&l=82
[5] https://cs.chromium.org/chromium/src/content/common/content_param_traits_macros.h?rcl=db55c1a8af9f153fc8c297fcc5735dabfecba4ee&l=55