Change V8VarConverter::FromV8Value to complete synchronously if possible

Change V8VarConverter::FromV8Value to complete synchronously if possible

Previously V8VarConverter::FromV8Value would always complete asynchronously
(we would always post a task to the message loop to run the callback). However,
this adds latency to the transmission of postMessage'd messages. It also has
the unfortunate consequence that posted messages can arrive out of order with
other messages (such as view changes). We should attempt to convert
synchronously if possible and only resort to asynchronous conversion
if needed.

BUG=367896
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=277374

[dmichael (off chromium), 2014-06-13 17:24:49]

I switched the bug from the PDF one to the sync postMessage API one. If you had
a more specific bug, feel free to move it to that.

Thanks for doing this, it should be a nice benefit to latency when we send
ArrayBuffer, Array, or Dictionary. I think this will also be a little cleaner
than the "FlushSync" thing I had in my CL.

lgtm!

https://codereview.chromium.org/331593002/diff/40001/content/renderer/pepper/...
File content/renderer/pepper/message_channel.cc (right):

https://codereview.chromium.org/331593002/diff/40001/content/renderer/pepper/...
content/renderer/pepper/message_channel.cc:297:
V8VarConverter::VarConversionResult conversion_result =
Being nested, maybe it would be enough to call it "Result"? Anyway, I think it
would be good to have a different name from the local VarConversionResult to
avoid confusion.

[raymes, 2014-06-16 02:59:25]

https://codereview.chromium.org/331593002/diff/40001/content/renderer/pepper/...
File content/renderer/pepper/message_channel.cc (right):

https://codereview.chromium.org/331593002/diff/40001/content/renderer/pepper/...
content/renderer/pepper/message_channel.cc:297:
V8VarConverter::VarConversionResult conversion_result =
Done. Well caught - I didn't notice the local class :) I called it VarResult to
indicate that a Var is being returned in it.

[raymes, 2014-06-16 02:59:30]

The CQ bit was checked by raymes@chromium.org

[commit-bot: I haz the power, 2014-06-16 02:59:44]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/raymes@chromium.org/331593002/60001

[commit-bot: I haz the power, 2014-06-16 07:23:41]

Change committed as 277374