Partial revert "Make IsSyncRequested false by default"

This reverts commit 8c872e02bbc75b817e036ed714ae652e3f638fa8.

Reason for revert:
https://bugs.chromium.org/p/chromium/issues/detail?id=973770

Note: This CL does not revert the changes the the test files as
instructed by treib@ who is the author of the original CL.


Original change's description:
> Make IsSyncRequested false by default
>
> Before this CL, the value of IsSyncRequested was essentially undefined
> while Sync was disabled: It was true by default, but false if the user
> had previously enabled Sync and then disabled it again.
> This CL resolves the inconsistency by making it false by default.
>
> Bug: 906034
> Change-Id: I2ce92f5df5ebb38a34ebde43d471839ad1184961
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1508453
> Reviewed-by: Jan Krcal <jkrcal@chromium.org>
> Commit-Queue: Marc Treib <treib@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#642447}

TBR=treib@chromium.org,jkrcal@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 906034, 973770
Change-Id: I501f563cae8ff63de21cd66d933281e0af9542ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1658571
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Kariah Davis <kariahda@chromium.org>
Cr-Commit-Position: refs/branch-heads/3809@{#307}
Cr-Branched-From: d82dec1a818f378c464ba307ddd9c92133eac355-refs/heads/master@{#665002}

• chrome/browser/sync/test/integration/single_client_standalone_transport_sync_test.cc[diff]
• chrome/browser/ui/webui/sync_internals_browsertest.js[diff]
• components/sync/base/sync_prefs.cc[diff]
• components/sync/base/sync_prefs_unittest.cc[diff]
• components/sync/driver/profile_sync_service_startup_unittest.cc[diff]