Move ChromeBrowserMainParts code out of chrome_content_browser_client.cc

Currently, ChromeContentBrowserClient::CreateBrowserMainParts() does all
the work of selecting the right parts to instantiate. Move this code
into a new ChromeBrowserMainParts::Create() instead. Then
chrome_content_browser_client.cc, which is the one of the largest and
slowest to compile files in the source tree, builds about 3 seconds
faster in build time benchmarks on a physical workstation. While
chrome_browser_main.cc, which compiles in about half the time, builds
about 2.5 seconds slower. Given modern CPUs with many cores, this better
balances parallel work. Since compiling chrome_content_browser_client.cc
blocks the critical path to linking the chrome binary, this speeds up
the overall build by about 3 seconds.

Change-Id: I4223aa5fc88f615b5157397fffdc47a8b88a7d2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7022412
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1527728}