[renderer]: Don't call blink::WebCache::pruneAll() if blink is not initialized.
[renderer]: Don't call blink::WebCache::pruneAll() if blink is not initialized.

The OnMemoryPressureListener can happen at any time after it is registered,
which may be before Blink is initialized. If this happens then pruneAll() will
call WTF::current_time(), which calls the uninitialized currentTimeFunction,
causing a crash.

BUG=447208
TBR=kerz@chromium.org

Review URL: https://codereview.chromium.org/933713002

Cr-Commit-Position: refs/heads/master@{#316619}
(cherry picked from commit 7fbb3bd568af0218bbddec79a7ae83cc3878ca3e)

Review URL: https://codereview.chromium.org/929423002

Cr-Commit-Position: 665a5787e4610f1ec55d3a4e2f02ab0724e38987@{#312}
Cr-Branched-From: 4cd5f18d847990599feba509541d8a7b9a16883a@{#310958}

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