Diagnostics: fix incorrect reference to storage

The IDS_DIAGNOSTICS_NOT_ENOUGH_AVAILABLE_MEMORY message is shown when
there is insufficient free RAM to run a test, not free disk space.
Remove the reference to storage management because that is wrong
(freeing disk space will not help in this case), and replace it with a
suggestion to close tabs and apps as appears in the similar
IDS_DIAGNOSTICS_NOT_ENOUGH_AVAILABLE_MEMORY_CPU message.

Updated UI screenshot: https://storage.cloud.google.com/chromium-translation-screenshots/28e1e821c9792e25490d0064fd00db607b0709cf

Fixed: b:323099753
Change-Id: Ic8f33bda939ac9783a167d9c1d237b27489e41e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5256817


Commit-Queue: Peter Marheine <pmarheine@chromium.org>
Reviewed-by: Michael Checo <michaelcheco@google.com>
Reviewed-by: Ashley Prasad <ashleydp@google.com>
Cr-Commit-Position: refs/heads/main@{#1282195}