Transform JPEGImageEncoder to handle data race problem in jpeg initialization

There's a data race problem in chromium_jpeg_make_c_derived_tbl (Issue 581542).
Due to potential complexity of changing an upstream library, we enforce the
execution of chromium_jpeg_make_c_derived_tbl to happen only on the main thread
by splitting the JPEGImageEncoder::encode function into two parts: the first
part deals with initialization of jpeg struct (that wraps the problematic
chromium_jpeg_make_c_derived_tbl), the second part deals with the rest of
encoding process. We post the first part as a task on main thread and leaves
the second part on the Background thread pool.

This transformation of JPEGImageEncoder happens to be part of the efforts of
another Issue (Issue 571398: progressive image encoding for JPEG).

BUG=581542,571398

Review URL: https://codereview.chromium.org/1684943002

Cr-Commit-Position: refs/heads/master@{#376211}

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