Fix GN label names for blink_heap_unittests, blink_platform_unittests.

The GN convention for test binaries is to have the label name
match the output name where possible, so this patch renames
:heap_unittests to :blink_heap_unittests and :platform_unittests
to :blink_platform_unittests.

This fixes various build errors encountered in flipping the blink
bots to GN.

TBR=brettw@chromium.org
BUG=432959

Review URL: https://codereview.chromium.org/1516683003 .

Cr-Commit-Position: refs/heads/master@{#364246}

• BUILD.gn[diff]
• testing/buildbot/gn_isolate_map.pyl[diff]
• third_party/WebKit/Source/platform/BUILD.gn[diff]
• third_party/WebKit/public/BUILD.gn[diff]