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This fixes the Accept-Language request header when the Network Service
is enabled by adding back q-values for language entry (to properly
denote user preference).

No change to legacy code path.

Bug: 968287
Test: out/.../bin/system_webview_shell_apk launch 'https://httpbin.org/headers' (verify header has q-values)
Test: run_webview_instrumentation_test_apk --enable-features=NetworkService,NetworkServiceInProcess -f AcceptLanguageTest#*
Cq-Include-Trybots: luci.chromium.try:android_mojo
Change-Id: Ideb10bdf299505b40d96df8c771509a11c102ffc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1637451Reviewed-by: Changwan Ryu <changwan@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664895}