Forked from issue 919123.
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Repro:
1. Open https://www.ecma-international.org/ecma-262/9.0/index.html
2. Wait for the page to load completely
3. View the page source (via the right-click menu or Ctrl-U key)

Expected: the first screenful of the source is shown instantly, the rest loads in background
Observed: an empty tab for 5-15 seconds then the source is shown 

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Bisected to https://chromium.googlesource.com/chromium/src/+log/97d3d992..4ceae8dc
Suspecting r321074 "WebKit" https://chromium.googlesource.com/chromium/blink/+log/2f002c7..c4c009c

Specifically 13118c059bf843ba334e99aa296b01557c1cd0e8 = crrev.com/1011913006
"Remove ResourceFetcher's direct knowledge of Document, DocumentLoader, and Frame"
Landed in 43.0.2337.0

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The reason to suspect it is that the instantaneous display was possible only after the page finished loading.
Assuming the suspected CL is WAI, it'd be still nice if the source-viewer renders the text sooner.
With throttling, of course, so that the re-layout doesn't occur too frequently.
Theoretically related: bug 920571.