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There are 3 types of tasks in ExecuteCodeFunction that require FILE thread:
a) Reading the file content (for non-component extension files)
b) Retrieving the file GURL of the script file from base::FilePath
c) content l10n (if the script is CSS)

This CL does all of these in on UI->FILE hop and doesn't require
additional FILE thread hop.
Previously, the most common case, i.e. chrome.tabs.executeScript()
with {file:...}, would require UI->FILE->UI->FILE->UI. With this CL
this become UI->FILE->UI.

This CL accomplishes this with adding an optional callback to FileReader,
to specify additional tasks to be run on FILE thread.

So the thread hop changes are:
Component extension's executeScript() with file:... url would stay same.
*All* other means of executing script file:... url, e.g.
chrome.tabs.executeScript(,{file:..}) would require two (UI->FILE->UI) less
thread hops, whee!

BUG=622464
Test=None, internal only change.

Review-Url: https://codereview.chromium.org/2301713002
Cr-Commit-Position: refs/heads/master@{#419023}